Mask and components thereof

ABSTRACT

A patient interface assembly includes a cushion and frame assembly with a cushion portion and a frame portion that together form a breathing chamber. The frame portion includes an anterior aperture configured to receive the flow of breathable gas. Headgear includes a back portion, a pair of upper headgear straps extending from the back portion, and a pair of lower headgear straps. The patient interface assembly also includes a shroud in the form of a contoured plate. The shroud is removably mounted on the frame portion by way of a snap-fit connection and includes a pair of lower headgear connectors. The patient interface assembly also includes a forehead support extending from the shroud and having a free end with an upper headgear connector. The forehead support has a first curvature that curves in a first direction around an axis that is offset from the patient interface assembly and extends along a length of the forehead support.

CROSS-REFERENCES TO RELATED APPLICATIONS

This is a continuation of U.S. application Ser. No. 14/471,525, filedAug. 28, 2014, which is a continuation of U.S. application Ser. No.12/461,448, filed Aug. 12, 2009, now U.S. Pat. No. 10,307,554, which isa continuation of U.S. application Ser. No. 10/533,928, filed Jul. 29,2005, now U.S. Pat. No. 8,490,623, which is a U.S. national phase ofinternational application PCT/AU2003/01471, filed Nov. 6, 2003, whichdesignated the U.S. and claims benefit of U.S. Application No.60/424,005, filed Nov. 6, 2002; U.S. Application No. 60/447,327, datedFeb. 14, 2003; U.S. Application No. 60/488,752, dated Jul. 22, 2003; andU.S. Application No. 60/503,896, filed Sep. 22, 2003, each of which isincorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

The invention relates to a full-face mask for use with Non-InvasivePositive Pressure Ventilation (NIPPV), Continuous Positive AirwayPressure (CPAP) and ventilators generally.

The delivery of a supply of breathable gas at positive pressure to apatient from a ventilator requires some sort of interface betweenmachine and patient. An endo-tracheal tube is typically used as apatient interface in invasive ventilation. In non-invasive ventilation,some form of mask is used as a patient interface.

A mask typically comprises a chamber having a nose-receiving cavitydefined by a shell or frame. The mask typically further comprises acomfortable face-contacting portion, such as a cushion, which may besecured to an edge of the shell or frame. Masks are typically held inposition on a patient's face using an arrangement of headgear, such as aset of elastic straps. It is a continuing challenge for mask designersto improve the comfort of masks, particularly where the mask has to beworn for many hours.

Unless a mask is constructed for each user, because of the wide varietyof shapes, most designs of masks represent a compromise. One design ofmask might be a good fit for a sub-group of patients with one shape ofnose (e.g., with a high nasal bridge), but poorly fit another sub-groupwith a different shape of nose (e.g., with a low nasal bridge). It canbe particularly difficult to design a mask which provides a good seal inthe nasal bridge region because that region of the face is particularlysensitive.

Folds and creases in the mask cushion can become very uncomfortable on apatient's face with prolonged wear. Furthermore, in spite of the use ofa cushion, the edge of a mask frame can be felt through the cushion andpresent an uncomfortable surface to the patient's face, particularly ifthe cushion is compressed.

In some cases it is appropriate for a mask to include a vent whichamongst other things can allow a controlled leak flow of gas from themask to prevent a build up of CO₂ within the mask. There may also beinadvertent or unintentional leak from the mask, for example, at ajunction between the mask and the patient's skin. The functioning ofsophisticated control algorithms in ventilators, particularly thoseresponding to a respiratory flow signal, is improved with the use of amask which provides low or zero unintentional leak flow.

Patients move during sleep. In addition, the shape of their head canchange during sleep, due to, for example, swelling. While a mask may fita patient well when initially fitted, because of such movement, the maskmay not fit well later in the night. Prior art masks typically includeelastic headgear straps that can be shortened or stretched or otherwiserearranged on the head to return the mask to a comfortable low-leakposition.

The level of pressure support provided by the ventilator can vary duringthe course of treatment. Some Continuous Positive Airway Pressure (CPAP)devices provide an initial ramp from a low pressure up to a therapeuticpressure. Other CPAP devices automatically adjust the pressure inaccordance with indications of flow limitations. Other devices vary thelevel of pressure support within a respiratory cycle of the patient, forexample, by providing a higher level during inhalation and a lower levelduring exhalation. Elastic headgear straps must be arranged to suit thelevel of pressure. If the elastic straps are arranged to suit a highpressure level, there is a risk that the straps will be too tight anduncomfortable for a low pressure level.

BRIEF SUMMARY OF THE INVENTION

It is an aspect of the invention to provide a comfortable low-leak maskfor use with Non-Invasive Positive Pressure Ventilation that overcomesthe limitations of prior art masks.

In another aspect, it is desirable to provide a mask system that has oneor more of the following features, each of which may assist withimproving patient compliance and/or treatment: headgear including strapsthat are substantially inextensible and/or micro-adjustable; and/or amask and/or cushion that includes various structures to allowenhanced/tailored sealing and/or fit at selected locations on thepatient's face.

In the description that follows, the following anatomical terms may beused:

-   -   Cephalic: In the direction of a vector running from feet to        head, and beyond. The nose is cephalad to the lips and chin.    -   Caudal: In the direction of a vector running from head to feet,        and beyond.    -   Anterior: In the direction of a vector running from the back of        the body to the front of the body, and beyond. The nose is        anterior to the ears, and the mask is anterior to the nose.    -   Posterior: In the direction of a vector running from the front        of the body to the back. The ears are posterior to the nose.    -   Coronal plane: A plane parallel to the plane containing the        head, feet, and tips of the shoulders. A strap passing from the        left ear, over the top of the head, to the right ear would be a        coronal strap.    -   Sagittal plane: A plane parallel to a plane passing through the        head, feet, back of the spine, and tip of the nose.    -   Nuchal: Pertaining to the (muscles of the) back of the neck.    -   Occipital: Pertaining to the bony prominence where the muscles        at the back of the neck insert into the back of the base of the        skull.    -   External auditory meatus: Ear hole.    -   KgF: Kilograms force.    -   Zygoma: The roughly half-apricot sized anterior protrusion of        the cheekbone (Strictly body of zygoma).    -   Inner canthus: The point where the upper and lower eyelid meet        next to the bridge of the nose.

These and other aspects will be described in or apparent from thefollowing detailed description of illustrated embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

The illustrated embodiments will be described in relation to thefollowing drawings; wherein like reference numbers may refer to likeparts, in which:

FIGS. 1-5 illustrate a first embodiment of the present invention;

FIGS. 5A and 5B illustrate an alternative embodiment of the presentinvention;

FIGS. 5C-5H illustrate an alternative embodiment of the presentinvention with micro adjustability and quick-release capability;

FIGS. 6-15C schematically illustrate a mechanism and principles thereoffor changing strap tension in accordance with air pressure supplied tothe patient;

FIGS. 16-35 illustrate an alternative embodiment of the invention;

FIGS. 36-40B illustrate an embodiment of the present invention in whichthe sides of the patient's nose can be effectively sealed;

FIGS. 41-53 illustrate alternative embodiments of the present inventionshowing frames/cushions enabling enhanced sealing along the sides of thepatient's nose;

FIGS. 53A-G illustrate further embodiments of a frame in which fins areprovided to support the cushion;

FIG. 53H illustrates an additional embodiments of the present inventionin which the frame includes a pad;

FIG. 53I illustrates an exploded perspective view of yet anotherembodiment of the present invention;

FIGS. 53J-53P illustrate yet another embodiment of the present inventionin which the frame supports an inflatable cushion;

FIGS. 54A-54C are rear elevation, side elevation and bottom plan views,respectively, of a prior art ACLAIM cushion in exploded view;

FIG. 54D is a cross section of the prior art ACLAIM cushion of FIGS.54A-54C;

FIGS. 55A-55C are rear elevation, bottom plan, and side elevation views,respectively, of a cushion assembly according to a first embodiment ofthe present invention;

FIG. 55D is a cross section of the cushion assembly according to thefirst embodiment;

FIGS. 56A-56F are rear elevation, top plan, bottom plan, side elevation,rear perspective, and front perspective views of a flexible element ofthe cushion assembly according to the first embodiment;

FIGS. 57A-57C are graphical illustrations of mechanical properties ofthe cushion assembly according to the first embodiment, a MIRAGE®cushion, and an ACLAIM cushion, respectively;

FIG. 58 is a cross section of a cushion assembly according to a secondembodiment of the present invention;

FIGS. 59A-59E are front elevation, rear elevation, side elevation, frontperspective, and rear perspective views of a flexible element of thecushion assembly according to the second embodiment;

FIGS. 60A-60D are graphical illustrations of mechanical properties ofthe cushion assembly according to the second embodiment, a MIRAGE®cushion, an ACLAIM cushion, and a comparison of the mechanicalproperties of the three cushions, respectively;

FIG. 61A-61E are graphical representations of the operation of thecushion assemblies according to the first and second embodiments under acompressive force;

FIG. 62 is a perspective view of a cushion assembly according to a thirdembodiment of the present invention;

FIGS. 63A-63E are front elevation, rear elevation, side elevation, frontperspective, and rear perspective views, respectively, of a flexibleelement of the cushion assembly according to the third embodiment;

FIGS. 64A-64E are front elevation, rear elevation, side elevation, frontperspective, and rear perspective views, respectively, of a retainer ofthe cushion assembly according to the third embodiment;

FIG. 65 is a cross section of a cushion assembly according to a fourthembodiment of the present invention;

FIG. 66 is a cross section of a cushion assembly according to a fifthembodiment of the present invention;

FIG. 67 is a cross section of a cushion assembly according to a sixthembodiment of the present invention;

FIGS. 68 and 69 illustrate an embodiment of the present invention inwhich the stiffness of the cushion can be selectively varied;

FIGS. 70-79 illustrate further embodiments of cushions according to thepresent invention;

FIG. 80 illustrates a cross-sectional view of still another embodimentof the present invention;

FIG. 81 is a relaxation curve for a foam suitable for use as a flexibleelement according to all embodiments of the present invention; and

FIG. 82 is a schematic illustration of an aspect of the technology withan active tensioning element.

FIG. 83 is a schematic illustration of an aspect of a chassis with arib.

DETAILED DESCRIPTION OF THE INVENTION

The following detailed description of the invention includes disclosureof a number of different features which are applied to variousembodiments of mask assemblies. It is to be understood that any featuredescribed in relation to one embodiment may be used in conjunction withone or more features in another embodiment.

Headgear A. Inextensible Straps

FIGS. 1-5 show one embodiment of a mask system, including a maskassembly 15 and a headgear assembly 20. As shown in FIG. 1, the headgearassembly 20 includes a plurality of straps that are joined together atjoints and are configured and arranged so as to substantially surroundthe patient's head. These straps are connected to the mask assembly 15to thereby retain the mask assembly 15 in relation to the patient'sface. The mask assembly 15 is shown merely as an example to demonstratethe application of the headgear assembly 20. The mask assembly 15 may besubstituted by any suitable respiratory mask, as would be apparent toone of ordinary skill in the art.

To retain the mask assembly 15 in position, the headgear assembly 20utilizes a sagittal strap 25 and a horizontal strap 30. The horizontalstrap 30 is arranged generally horizontally and is wrappedcircumferentially around the patient's head. Each end 31 of eachhorizontal strap 30 is coupled to the mask assembly 15. The arrangementbetween the horizontal strap 30 and the mask assembly 15 will bediscussed in further detail below. The horizontal strap 30 is preferablyarranged to pass just inferiorly to each ear and across the insertionarea of the neck muscles into the base of the skull which is generallyindicated at 36 in FIG. 2.

A posterior end 40 of the sagittal strap 25 is provided generally at amidpoint of the horizontal strap 30 so as to be positioned at anintermediate posterior area of the patient's head. As can be seen inFIG. 1, the width of the sagittal strap 25 at the posterior area of thepatient's head is relatively wide, e.g., about twice the width of theremaining portions of the sagittal strap 25. This increase in surfacearea is advantageous as it helps to prevent the strap from sinking intoa very fatty or compliant back portion of the patient's head as pressurechanges, or as strap tension changes. Of course, the strap 25 should notbe so wide and/or thick that it becomes uncomfortable. The strap may bemade from a cool material, such as BREATHOPRENE™.

The sagittal strap 25 extends from the horizontal strap 30, e.g., theposterior end 40 f, across the vertex of the skull, generally indicatedat 45, and extends generally interiorly across a forehead of thepatient's head, generally indicated at 50 (see FIG. 2). The sagittalstrap 25 has an anterior end 55 coupled to the mask assembly 15, as willbe discussed in further detail below.

It may also be preferable for the headgear assembly 20 to include a pairof coronal straps 35 that interconnect the sagittal and horizontalstraps 25, 30. A superior end 60 of each coronal strap 35 is connectedto the sagittal strap 25 proximate the vertex 45 of the patient's head.Each coronal strap 35 extends from the vertex 45, e.g., the superior end60, laterally and inferiorly across the head and connects to thehorizontal strap 30 just anteriorly to and just inferiorly to each earat inferior ends 65 of the coronal straps 35.

Each inferior end 65 of the coronal straps 35 may be connected to thehorizontal strap 30 via stitching and/or an adhesive. Alternatively, thehorizontal strap 30 can be connected with both the coronal straps 35and/or the sagittal strap 25 with one or more clip elements which willallow adjustability between one or more of the strap portions.Alternatively, it is possible that one or more of the straps of theheadgear assembly 20 may be formed from a single piece of material.

To maintain a secure and comfortable fit of the mask assembly 15, thestraps of the headgear assembly 20 are preferably formed to besubstantially inextensible. Stated differently, the straps may besomewhat flexible, however, the straps are preferably not capable ofsignificant elongation. The straps have sufficient stiffness or rigidityto retain their shape. Contemplative materials for the straps includepolyvinylchloride (PVC), leather, polypropylene, or polyurethane. Othermaterials are, of course, possible. For example, another contemplatedsuitable material may be a relatively strong cloth tape. It is alsocontemplated that the straps may be lined with a felt material to add adegree of comfort to the patient. Other alterations may includeperforations or holes to allow cooling through the straps.

B. Micro-Adjustment of Straps 1. First Embodiment

Referring to FIG. 1, the headgear assembly 20 is coupled to the maskassembly 15, preferably in a manner so as to allow adjustment of theposition of the mask assembly 15 relative to the straps of the headgearassembly 20. The mask assembly 15 includes a frame assembly 70, acushion 75 to interface or make contact with the patient, and a cushionsupport 80 interposed between the frame assembly 70 and the cushion 75.The cushion support 80 includes an aperture (not shown) by whichpressurized air is provided to a pressurized chamber of the maskassembly 70, which is delivered to the airways of the patient.Typically, an elbow 85 is releasably connected to the aperture of thecushion support 80. The swivel elbow includes a quick release connector86 that is provided to an air delivery tube (not shown) which in turn iscoupled to an air delivery device, e.g., a flow generator (not shown).

The frame assembly 70 includes a chassis 95 (best shown in FIG. 3)provided with one or more cross members 100. The cross members 100support a cantilevered extension 90 which is coupled to the anterior end55 of the sagittal strap 25. The anterior end 55 of the sagittal strap25 is provided with a threaded portion 105 that is guided through areceiving aperture 91 provided on the extension 90, as best shown inFIGS. 1 and 2. A nut 110 can be rotated about the threaded portion 105to thereby adjust the distance between the extension 90 and the forehead50 of the patient. Accordingly, the strap tension can be finely adjustedespecially if the sagittal strap 25 is made of a substantiallyinextensible material, as described above.

FIG. 3 shows a front view of the mask assembly 10. In FIG. 3, it can beseen that the chassis 95 includes a plurality of finger portions 115extending away from the chassis 95. The lower most finger member 115 oneach side of the chassis 95 includes an aperture 120 configured toreceive a threaded portion 105 extending from each end 31 of thehorizontal strap 30. A nut 110 is threadedly secured to the threadedportion 105 so that the distance between the mask assembly 15 and theface of the patient can be finely tuned. Accordingly, the straps can betightened to a high degree of accuracy so that the forces applied to theface are appropriate over a given pressure range, from about 2 to 40cmH₂O.

As best shown in FIGS. 3 and 4, the threaded portion 105 has asubstantially rectangular cross section, including two relatively flatsides and two sides having threaded sections. The apertures 91 and 120may have a shape that is relatively complimentary to the shape of thethreaded portion 105. For example, the receiving apertures 91, 120 mayhave a substantially rectangular shape to thereby prevent rotation ofthe threaded portion 105 when adjusting the nuts 110. This. Although notshown, the end of the threaded portion 105 may also include an element,e.g., a member with a rectangular aperture, to help prevent rotation ofthe threaded portion 105.

FIG. 4 shows a side view and more clearly shows the connection betweeneach horizontal strap 30 and the frame assembly 70. In addition, FIG. 4shows that each of the finger portions 115 is movably, e.g., pivotably,connected to transverse portions 96, 97 of the chassis 95. In thisparticular example, the top two finger portions 115 are interconnectedwith a cross bar 125 while the bottom two finger members 115 areconnected with a similar cross bar 130. Accordingly, the top two fingermembers and the bottom two finger members, on each side of the chassis,respectively, can move in unison, which may be advantageous from theperspective of force distribution. However, it is contemplated that eachof the finger members can be independently movable with respect to thetransverse members 96, 97 of the chassis 95.

In this example, the threaded portion 105 which extends from the end 31of each horizontal strap 30 is threaded through the receiving aperture120 which is provided to the lower two finger portions 115. As such, asthe nut 110 is tightened, any slack which is left in the horizontalstrap 30 will be taken up. When all of the slack is taken up, anyfurther tightening of the nut 110 will cause the lower two fingerportions 115 on the right hand side to rotate in a clockwise sense (asviewed from above) against the cushion support 80. The lower twoportions on the left hand side will rotate in a counter-clockwise sense,as viewed from above. The cushion support 80 in at least the lateralportions 82 adjacent the finger portions 115 is flexible. Due to thisflexibility, the lateral portions 82 impose a force on the correspondingsection of the cushion 75 to thereby pinch against the sides of the noseof the patient.

As shown in FIG. 4, the cushion 75 is integrated with the cushionsupport 80 is flexible or deformable, for example by the fingers 115, inorder to better fit the contours of the individual face. There is atrade-off between making the cushion support very flexible to allowbetter fitting of the face, versus so extremely flexible that theinternal volume of the mask changes excessively (e.g., >20 mL) with eachbreath, which would make measurement of tidal volume difficult. Atypical silicone of 1-3 mm thickness is suitable.

FIG. 5 shows a rear view of the mask assembly 15 in which the upper twofinger portions 115 on the left side of the patient's face are manuallypushed in against one of the lateral portions 82 of the cushion support80. The force which is applied from the upper two finger portions 115 tothe lateral portion 82 of the cushion 80 causes deformation of thecushion 75 such that it pinches against the side of the nose, therebyaccommodating differently shaped noses and enhancing seal performance ofthe cushion 75.

In FIGS. 1-5, tension in the straps, along with flexibility of thelateral sides 82 of the support 80, causes the fingers to rotate andpinch together thereby squeezing the sides of the nose and possibly aportion of the patient's face. Accordingly, any irregular facestructures can be accommodated by the independent rotating capability.This also helps to evenly distribute the load on the face, therebyrelieving areas of high contact force.

FIG. 5 shows the effect of increasing tension in the top strap (notshown), which results in pinching in the nasal bridge region of thepatient. This is particularly useful for bi-level treatment so that atlow pressures only low forces are applied and at high pressures highforces are applied, which is helpful for improved comfort and sealing.The provision of headgear made of an inelastic material helps prevent“pistoning” of the mask on the face, that is to say lifting off the faceat high pressure, and/or digging into the face at low pressure duringbi-level treatment.

In the embodiment of FIGS. 1-5, the two upper finger portions 115provided on each transverse portion 96, 97 of the chassis 95 are notshown as being connected to any strap member of the headgear assembly20. However, the positioning of such upper strap portions may be asshown by the imaginary lines 135, 140 in FIG. 2. In particular, theimaginary line 135 represents a situation where an upper strap portionwould be connected to a midsection of the coronal strap 35. Imaginaryline 140 represents a situation where an upper strap portion would beconnected to the horizontal strap 30 on each side of the headgear.

2. Second Embodiment

FIGS. 5A and 5B depict an alternative embodiment of the presentinvention in which a horizontal strap 30 includes a first connectorportion C1 that is selectively coupled with a second connector portionC2. The second connector portion C2 is attached to a lower strap LS andan upper strap US. The upper and lower straps LS, US may be tightened tothe point where they bear against the transverse portions 82 of theframe 70, thereby imparting an inward force on the cushion 75 to seallaterally against the sides of the patient's nose. The upper and lowerstraps LS, US can be adjustably fixed to the frame 70 (as indicated bythe arrows) to best position the area where the inward force will beapplied. In this embodiment, the lateral portions 82 of the support 80are made of flexible material, while the apex is more rigid.

3. Third Embodiment

FIGS. 5C-5H illustrate yet another embodiment of the present inventionin which an adjustment mechanism allows coarse and fine adjustment ofthe head strap tension. FIG. 5C shows the overall mask assembly,including a mask frame 800 and which is provided with an elbow 805including an anti-asphyxia valve 810. A quick release clamp 815 isprovided to allow the patient to quickly remove the headgear, asdescribed in U.S. patent application Ser. No. 10/235,846, filed Sep. 6,2002, incorporated herein by reference in its entirety. FIG. 5D showsthe quick release mechanism in a partially opened position, while FIG.5E shows the quick release mechanism fully opened.

A strap 820 includes a pair of strap ends 820A, 820B provided to holdthe mask assembly on the patient's head. One of the strap ends, e.g.,820B may be releasably connected, e.g., via a slot 821, to one end ofthe mask frame in a fixed position, thereby avoiding variation in lengthof the strap 820 which could occur with repeated removal and re-placingof the mask assembly. The other strap 820A is positioned and configuredto be adjustable. Of course, both ends of the strap 820 may beadjustable. The strap end 820B may be looped through the slot 821 bycreating a loop in the strap 820 that is fixed, e.g., via a rivet 822 orother fastener.

An adjustment assembly 825 may be provided to adjust the strap 820. Inparticular, as shown in FIG. 5F, the adjustment assembly 825 may includea generally rectangular prism 830 having a strap-receiving slot 835through its entire length. The slot has an opening 840 shown in the endface of the prism. The prism has an upper portion 836, a lower portion837 and a threaded screw 845, best shown in FIG. 5G. The upper and lowerportions 836, 837 can assume a first position, adapted for coarseadjustment of the strap length, in which the strap end 820A can bepulled through the length of the slot 835. In a second position, adaptedfor fine adjustment, the upper and lower portions 836, 837 are broughttogether so that the threaded screw 845 engages with the strap end 820A,preventing its movement through the slot 835. The screw 845, uponrotation, translates the strap end 820A to tighten or loosen theheadgear. The upper and lower portions 836, 837 translate via a slot andpin arrangement 841, 842, to enable the screw 845 to move into and outof engagement with the slot 835. The threaded screw 845 has one endextending beyond the length of the prism 835 having a first gear portion850. As shown in FIG. 5H, the first gear portion 850 in turn engageswith a second gear portion 855 at right angles thereto. The second gearportion 855 has a cylindrical knob 860 attached to it. By rotating thecylindrical knob 860, the second gear portion 855 rotates, driving thefirst gear portion 850 and therefore the screw 845. Depending on thedirection of rotation, the strap is either pulled or pushed through theslot, thus enabling fine adjustment of strap length.

C. Inflatable Bladder—Raviolus and Occipital Pneumatic Pillow 1. FirstEmbodiment

Referring to FIG. 1, the headgear assembly 20 is illustrated asincluding a particular form of bladder which shall be referred to as a“raviolus” 145 provided along, e.g., the sagittal strap 25 of theheadgear assembly 20. The raviolus 145 is provided to apply a relativelyconstant strap force against the patient's face over an entire range ofmask pressures. The raviolus 145 is an active component that doespneumatic work to pull the mask onto the face at higher pressure. Theraviolus 145 is in communication with pressure in the mask via a smalldiameter silicone tube 150 attached to a port either on or in closeproximity to the mask assembly 15. In this example, the tube 150 isprovided to the elbow 85. Tension in the sagittal strap 25 is at leastpartially driven by flow generator pressure via the raviolus 145, whichcauses greater strap tension/displacement at higher pressures and lessstrap tension/displacement at lower pressures.

Although raviolus 145 is shown as the preferred embodiment, variation instrap tension/displacement can be achieved by other mechanisms,including electrical and mechanical systems. As the mask pressure rises,the raviolus pressure rises, causing the raviolus to inflate to a morespherical shape, shortening it anteroposteriorly and therefore pullingposteriorly on cantilever 90, thereby pressing the mask more firmlyagainst the face.

To a first approximation, the posteriorly directed force generated bythe raviolus or cantilever 90 is linear on mask pressure. The constantof proportionality is greater as nut 110 is tightened, causing theraviolus to be more elongated at any given mask pressure. Accordingly,the raviolus 145 can be considered an automatic compensating mechanismwhich if set so that the mask seals at one pressure it will seal at allpressures and it will constantly balance the air pressure in the mask.

Inflating the raviolus by volume ΔV as pressure rises by ΔP does workΔVΔP to pull the attachment point 91 on cantilever 90 backwards througha distance against a force.

Although the raviolus 145 is only provided on the top strap, otherscould also be provided on the remaining straps, including the horizontalstraps 30. However, no raviolus 145 is applied to the lower straps inthis embodiment since the natural tendency of the patient's cheeks andbottom lip to billow somewhat approximates the action of the raviolus145 to create a good seal in that area over the range of operatingpressures. In other words, the sealing mechanism for the top of the maskand the sealing mechanism for the bottom of the mask are different. Forthe bottom of the mask, the mask designer can rely on the bottom lip andcheeks of the patient to inflate whereas at the top of the mask adifferent mechanism is used because in part, the facial structure of thenasal bridge region is very bony and rigid.

2. More Details on Raviolus

Having explained the raviolus 145 in general terms, attention is nowdirected to FIGS. 6-15A which describe more specific principles of theraviolus in detail.

The raviolus 145 may be a rectangular thin walled tube of elastomer suchas silicone, pleated along two sides 147, and then sealed at thenon-pleated ends 150. The non-pleated sealed ends 150 are inserted intoa headstrap of the mask assembly 15, e.g., the sagittal strap 25. FIGS.6 and 7 show the raviolus 145 in a relaxed state, with little or nopressure, e.g., during expiration of the patient, while FIGS. 8 and 9show the raviolus 145 under treatment pressure, e.g., during inspirationof the patient. In relation to bilevel ventilation, where the patient isexposed to relatively higher pressure during inspiration and relativelylower pressure during expiration, inflating the raviolus 145 to pressurep, the raviolus 145 will shorten and/or widen, and the headstrap 25 willbe pulled tighter.

In the following, the raviolus 145 is assumed to be floppy in thelongitudinal direction and stiff transversely, so that it maintains theabove flat topped cross section at all pressures. This could be achievedin manufacture, for example, by gluing rigid rods transversely to thetop and bottom surfaces, or moulding the top and bottom surfaces to havetransverse ridges, and/or by using internal tie wires between the rightand left concertina walls. In practice, the basic idea and the followingdiscussion works to a loose approximation without these refinements.

In the example of FIG. 10, the raviolus 145 has a width W and aninflated height 2h. The concertina sides are assumed to exert negligibleforce and are not included in the calculations. The force in the strapis f. In FIG. 10, the raviolus 145 is sliced in half transversely, and apair of rigid plates, one of which is shown above as reference number155, are attached to the cut line. The two plates are connected by arigid rod 160. The force in the rigid rod 160 is also f.

Let the axial surface tension (force per unit length) in the top strapbe t. Because the assembly does not move with time, the forces acting onthe visible plate 155 must sum to zero. These forces comprise 2tW actingto the left, 2 pWh acting to the right, and f in the rigid rod 160acting to the right:2tW=f+2pWh  (eqn 1)

If there were no tension in the straps, for p>0, then the top and bottomsurfaces of the raviolus 145 would together form a cylinder (θ=π/2).When the strap 25 is under tension, the surface becomes two incompletesymmetrical segments of a cylinder of radius r, as shown in crosssection in FIG. 11.

The line where the two surfaces meet the strap is under equilibrium,i.e., has no net force on it. Because the surface of the raviolus 145beyond the attachment point is irrelevant, the universe beyond theattachment point can be replaced with the remainder of a cylinder ofradius r.

The cross section of the top or bottom surface is an arc of a circle,radius r, and subtending an angle 2θ at the center of the circle.

From simple geometry, the angle between the top or bottom surface of theraviolus 145 and the continuation of the strap is also θ, as shown inFIG. 12. Therefore, since the net force at the junction is zero, wehave:f=2wt cos(θ)  (eqn 2)

A further constraint obvious from FIG. 12 is:h=r(1−cos(θ))  (eqn 3)

Finally, if the raviolus 145 has a flattened length (distance betweenstraps) of L, then the circumference of the arc is given by:L=2rθ  (eqn 4)

Accordingly, there are four simultaneous equations, five unknowns p, h,t, f, and θ, and the constants W and L. Solving for f:f=pWL cos(θ)/θ(0<θ<π/2)  (eqn 5)

Note the following special features:

(i) If the raviolus is flattened, i.e., θ→0, then any positive pressuregenerates infinite force.

(ii) If θ=π/2, i.e., the raviolus is cylindrical, then f is zero for allp. Ignoring the behaviour of the concertina sides, W and L play an equalrole in force generation. Doubling either will double the force.

The force generated varies with the length of the raviolus 145. FromFIG. 11, it can be seen that the distance x between the ends of theraviolus is given by:x=2r sin(θ)  (eqn 6)and substituting r from equation 4 gives:x=L sin(θ)/θ  (eqn 7)Recall that:f=pWL cos(θ)/θ  (eqn 5)

Table 1 was plotted using the above equations. Column 2 of Table 1 showsthe length “x” of the raviolus (see FIG. 11), as a fraction of theresting length L, for various angles θ. Column 3 shows the force “f”generated (see FIG. 10) as a fraction of the product of pressure p,width W, and resting length L.

TABLE 1 θ (degrees) x/L = sin (θ)/θ f/p WL = cos (θ)/θ 0 1.00 infinite10 0.995 5.64 20 0.980 2.69 30 0.955 1.65 40 0.921 1.10 50 0.878 0.73760 0.827 0.478 70 0.769 0.280 80 0.705 0.124 90 0.637 0.000

FIG. 13 plots the force f (as a fraction of pLW) against the length x(as a fraction of the resting length L).

Differentiating equations 5 and 7 with respect to θ gives:df/dθ=−pWL[sin(θ)/θ+cos(θ)/θ²]  (eqn 5a)dx/dθ=L[cos(θ)/θ−sin(θ)/θ²]  (eqn 7a)and dividing 5a by 7a gives (for 0<θ<=π/2):df/dx=pW[cos(θ)/θ+sin(θ)]/[sin(θ)/θ−cos(θ)]  (eqn 8a)

In the limit as θ→0 (empty raviolus), the denominator goes to unity, butthe numerator goes to infinity, so the spring has infinite positivestiffness. For a fully inflated raviolus (θ=π/4) the stiffness is +4pWh/π. Table 2 adds the stiffness to the previous table.

TABLE 2 Distance between straps Force generated Stiffness (fraction(coefficient (coefficient θ (degrees) of maximum) of pWL) of pW) 0 1.00infinite infinite 10 0.995 5.64 592 20 0.980 2.69 78 30 0.955 1.65 25 400.921 1.10 11 50 0.878 0.737 6.2 60 0.827 0.478 3.9 70 0.769 0.280 2.880 0.705 0.124 2.1 90 0.637 0.000 1.57

Example

A practical raviolus might have:

W = 5 cm =0.05 meters L = 6 cm =0.06 meters P = 20 cmH₂O =1960 N/m²~2,000 N/m²

If the raviolus 145 is partially inflated and held between two rigidsupports, then the strap tension increases linearly with pressure.

For any given geometry, the force generated is proportional to theresting length and breadth of the raviolus.

For any given pressure, the force generated is infinite when theraviolus is at its resting length, and falls off very rapidlythereafter.

As an example, a 6 cm long by 5 cm wide raviolus connected to 20 cmH₂Ogenerates about 0.633 KgF when it is shortened by 0.5 cm, 0.300 KgF whenit is shortened by 1.0 cm, and 138 grams force when it is shortened by1.5 cm.

An effect of this very strong dependence of force on length is thattightening the headstrap with a screw will permit any desired force tobe generated at given pressure.

The mask assembly 15 is held onto the face at three points by two straps25, 30. Take the mask assembly 15 to be an isosceles triangle of base 12cm and height 12 cm, less two small triangles in the bottom corners ofheight 2 cm and base 2 cm. Thus the area of the mask is 70 cm².

At a pressure of 20 cmH₂O, the air pressure will be exerting 1400 gramsforce, and at 5 cmH₂O it will be only 350 grams. Assume that in order toseal, it is necessary for the straps to exert a force 30% higher thanthis, or 1820 grams.

Per FIG. 15, assume that 1) the centroid C is about 4.5 cm up from thebottom of the mask, or 7.5 cm down from the top; 2) the bottom strap isattached around 3.5 cm up from the bottom of the mask, or 1 cm below thecentroid; and 3) the top strap attaches to a long lever arm some 15 cmabove the centroid.

Because the bottom strap attaches about 15 times closer to the centroidthan the top strap, the bottom straps take 15/16 of the load, leavingonly 100 grams to be borne by the top strap.

There is a 6 cm long by 5 cm wide raviolus in the top strap. It isconnected to the mask by the tube 150. It will generate 100 grams forceat 20 cmH₂O when its length is reduced to 4.35 cm. Its stiffness at thislength and pressure is 0.232 Kg force per cm.

Suppose the raviolus 145 is in series with a stretchy headgear strap ofelastance E_(STRAP). The free ends of the stretchy strap and raviolusare fixed. The elastance of the total system will be the elastance ofthe headgear plus the elastance of the raviolus.

For example, suppose the 5 cm wide by 6 cm long raviolus is mounted inseries with a well-washed traditional ResMed® headstrap, with anelastance of 10 cm per KgF. The raviolus is at 20 cmH₂O, is 4.35 cmlong, and exerting 0.1 Kg as before.

The spring constant of the raviolus under these conditions is 0.232KgF/cm, so its elastance is 4.3 cm/Kg. Therefore total system has a(local) elastance of 14.3 cm/Kg. The elastance of the entire system isdominated by the traditional strap.

As another example, start with a 5 cm wide by 6 cm long raviolus, withpressure 20 cmH₂O. The length is therefore again x₀=4.35 cm, andgenerating a force of f₀=100 grams. The raviolus is again in series witha strap of elastance 0.1 Kg/cm, i.e., spring constant Kstrap=10 Kg/cm.The next step is to determine what happens when the pressure is reducedto 5 cm H₂O.

The equation for the force generated by the stretchy strap in terms ofthe length x of the raviolus will be:fstrap=f ₀ −Kstrap(x−x ₀)

Plotting this on the graph for force generated by the raviolus at 20cmH₂O and 5 cmH₂O, we obtain FIG. 15A.

The headgear will shrink, causing the raviolus to lengthen from 4.35 cmto about 4.8 cm, and instead of the tension in the strap reducing from0.1 Kg to 0.05 Kg as desired, it will decrease to only about 0.55 Kg.

With no raviolus, the unnecessary strap tension to be borne by thebridge of the nose would be 75 grams. With a substantially inextensibleor rigid headstrap, the raviolus, correctly adjusted, would reduce thisto zero. But with a very sloppy headstrap, the unnecessary strap tensionwould be 45 grams, or a bit over half of what it would be with noraviolus.

2. Second Embodiment

FIGS. 16-35 illustrate another embodiment of the present invention.

A mask and headgear assembly, generally indicated at 410, is shown inFIGS. 16-18 as installed on a model of a head. The mask and headgearassembly 410 comprises a mask assembly 412, headgear 414, and aninflatable bladder which takes the form of an occipital pneumatic pillow416 coupled to the headgear 414 to adjust the fit of the headgear 414.

The mask assembly 412 includes a mask body assembly 418 and a mask frame436 (which will be described in more detail below) that acts as asupport or “skeleton” for the mask body assembly. The mask body assembly418 has a generally triangular shape when viewed from the front, asshown in FIG. 27.

In one preferred form, the mask body assembly 418 comprises aface-contacting portion 420 and a body portion 422. The face-contactingportion 420 of the mask body assembly 418 is to provide a detailed fitwithout causing pain, discomfort or skin damage. In particular, theface-contacting portion 420 is designed to provide a seal around thebony parts of the nose. In order to avoid damage to the skin, it ispreferable if no portion of the face-contacting portion 420 exerts anaverage pressure on the face that is greater than the average facialcapillary blood pressure (typically about 25 mm Hg).

In general, the face-contacting portion 420 is contoured to pinch thesides of the nasal bone (above the nasal cartilage) and at the level ofthe inner canthus. The face-contacting portion 420 forms aninwardly-facing seal at the sides of the nasal bone. However, theface-contacting portion 420 is designed not to pinch the wings of thenose, either directly by pressing on the cartilages, or indirectly bypressing on soft tissues nearby. Furthermore, it is preferable that theface-contacting portion 420 not contact the eye, lashes, or tear ductmechanism at the inner canthus of the eye.

The face-contacting portion 420 of the mask body assembly 418 includesseveral major contoured features, which can be seen in the views ofFIGS. 27-31. A notch 424 is provided to accommodate the bridge of thenose (nasion). A rise 426 is provided to fit the frontal process of themaxilla bone. A second notch 428 is provided to accommodate the cheekbone (i.e., maxillary process of the zygomatic bone). Additionally, asecond rise 430 is provided to fit the jowl, and a third notch 431 isprovided to accommodate the mandibular arch.

In one preferred form, the face-contacting portion 420 is constructed ofa polyurethane foam covered by a silicone “skin” or sheet. It ispreferable if the silicone material is the softest (i.e., lowestdurometer value) material that can be made without a tacky or peelingcharacter. Typically, the silicone skin would be adhesively bonded tothe foam to prevent wrinkling of the skin relative to the foam.

The body portion 422 of the mask body assembly 418 supports the elbow430, anti-asphyxia valve, and gas washout vent. It permits relativelyfree distortion or bending of the mask body assembly 418 relative to theframe 436 of the mask assembly 412, and also acts as a locating andconstraining mechanism to prevent the frame 436 from sliding out ofplace. The mask body assembly 418 is shown in the plan view of FIG. 33Aand in cross-section in FIG. 33B. In one preferred form, the bodyportion 422 is silicone, is co-molded with the face-contacting portion420, and is contiguous with the face-contacting portion 422.

The mask assembly 412 includes a pressure plate or frame 436 whichtransmits the forces from the headgear to the cushion. As best shown inFIG. 21, the frame 436 is generally triangular in shape and comprises abase 438, side 440 and apex portions 442. The frame 436 is resilientlyflexible, allowing the frame 436 to wrap around the jaw and nose of apatient. The base 438 and apex 442 portions of the frame 436 aregenerally constructed so as to be more flexible than the comparativelyrigid side portions 440. The apex and base portions 442, 438 define alongitudinal axis L about which the frame 436 can resiliently flex, asshown in FIGS. 32 and 35. The resilient flexibility of the frame 436allows the mask assembly 412 to more precisely fit a wider range offacial shapes. For example, the same mask assembly 412 could be used onpatients with a narrow angular face (the so-called crocodile shape) asthose with a wider flatter face (the so-called panda shape).

The base portion 438 of the frame 436 is generally “C” or crescentshaped. The apex portion 442 is generally boomerang or chevron shaped.

The flexible apex 442 and base 438 portions may be constructed from 1 mmpolypropylene sheet approximately 2 cm wide. Each side portion 440 canbe constructed from a pair of similarly shaped pieces of aluminum 100mm×20 mm×1 mm. The frame 436 can be riveted together with 4 rivets, orjoined by another known technique, such as adhesive bonding.

The frame 436, which is shown in isolation in the view of FIG. 32,includes a forked bracket 444 mounted on each side portion 440. Eachbracket 444 is constructed of aluminum or another substantially rigidmaterial. There are a series of holes 446 along the length of the sideportions 440 which are adapted to receive a bolt to thereby secure thebracket 444. The angle of the bracket 444 with respect to the sideportion 440 is adjustable by loosening the bolt, adjusting the angle,and tightening the bolt. The position of the bracket 444 along the sideportion can be adjusted by securing within a different hole. Bothbrackets 444 need not be mounted in the same relative position along theside portions 440. In this way, some allowance can be made for anyasymmetry in a patient's face. One bracket 444 is secured to each sideportion. The bracket 444 is adapted to receive and engage the nut 448 ofthe threaded arm 450 of the headgear 414.

In addition, the frame 436 may include a wedge shaped spacer. In use,the spacer is operatively secured between the side 440 and/or apex 442portions and the face-contacting portion 420 of the mask body assembly418. The spacer is 1-2 cm thick at the top tapering to zero about halfway down the mask. In addition, the wedge tapers to zero thickness fromoutside to inside. The wedge is constructed from a generallyincompressible material. The wedge provides additional force to the topof the mask body assembly 418 to assist in sealing. In addition, thewedge pinches the mask body assembly 418 at the sides of the nasalbones, pressing harder on the outside edge of the mask body assembly 418than on the inside.

The headgear 414 comprises a strap assembly 452, the occipital pneumaticpillow 416, or other active adjustable tensioning element, and the pairof threaded arms 450 that connect with the mask assembly 412. Ingeneral, the headgear 414 is constructed and arranged so that the forcevector from the mask assembly 412 to headgear 414 which originates atthe pneumatic center of the mask assembly 412 should pass through apoint midway between the right and left external auditory meatus.

As shown best in FIGS. 22 and 34, the strap assembly 452 comprises asub-occipital strap 456, a coronal (crown) strap 458, and a pair of earpieces 460. In this embodiment, the straps 456, 458, 460 are ends of asingle-piece headgear assembly, but in other embodiments of theinvention, they may be unitary straps, optionally connected together atappropriate points. The straps 456, 458, 460 may be constructed from aflexible but generally inextensible plastic material, such as 1 mmpolypropylene sheet, optionally covered on one or both sides with layersof foam, felt, or other cushioning material to increase comfort. Becausethey are formed of a flexible but inextensible material, the straps 56,58, 60 can conform to the shape of a patient's head, but they would notgenerally extend more than 1-2 mm when subject to 2 KgF tension.

The sub-occipital strap 456 passes under the occiput but above thenuchal muscles and is approximately 4 cm in width. The crown strap 458passes over the crown of the patient's head and is approximately 2 cm inwidth. The ear pieces 460 may be constructed so as to partially or fullysurround the ears. The ear pieces 460 may be constructed from anelliptical annulus of plastic material, generally 2 cm in width, andlined with skin contact grade felt, which should slightly overlap theannulus to prevent cutting into the root of an ear.

A pair of rigid threaded arms 450 extend from the ear pieces 460. In oneembodiment, they are constructed from 5 mm threaded nylon rod. The arms450 are arranged such that they are operationally proximate to theexternal auditory meatus and extend forwardly thereof in anapproximately horizontal plane. There is a barrel nut or thumb-wheel 468screwably mounted on and moveable along the length of each arm 450. Eachnut or thumb-wheel 468 is adapted to releasably engage with the brackets444 mounted on the mask frame 436, as shown in FIGS. 16, 17, 19 AND 20.The arms 450 are connected to the ear pieces 460 of the headgear 414 bya pivoting connector 470, as shown in FIGS. 19, 23 and 24. (First andsecond positions of the pivoting connector 470 are shown in FIGS. 24 and25, respectively.) In this way, the relative angular position of thearms 450 is adjustable. By moving the nut 468 along the threaded arm,the tension within the frame 436 and headgear 414 can be preciselyadjusted. This arrangement also provides a quick release mechanism.Since the frame 436 includes flexible base 438 and apex 442 portions, itwill flex in accordance with the position of the nuts 468 along the arms450 as they adjusted to find the most appropriate fit for the patient.

The inflatable occipital pneumatic pillow 416 is in force-transmittingrelationship with the straps 456, 458, 460 and is operationallypositioned under the strap assembly 452 and at the rear of the head,generally in the region of the occiput. The occipital pneumatic pillow416 can be inflated and deflated. In one embodiment, as shown in FIG.23, the occipital pneumatic pillow 416 is connected to an air deliveryconduit 462 of the mask assembly 412 via a tube 464. In this way, thepressure in the occipital pneumatic pillow 416 is similar to thepressure in the mask assembly 412. When mask assembly 412 pressureincreases, the occipital pneumatic pillow 416 is inflated, which resultsin a concurrent increase in headgear 414 tension and prevents the maskassembly 412 from lifting off the face and leaking. In this way movementof the mask assembly 412 during different mask pressures is dampened.

The occipital pneumatic pillow 416 is designed to have sufficient area(A_(bladder)) so that in conjunction with the pressure of air in theoccipital pneumatic pillow 416 (P_(bladder)), it will counterbalance theforce on the headgear straps 456, 458, 460 (i.e., the force on thestraps 456, 458, 460 caused by the pressure in the mask assembly 412).Generally, the area of the occipital pneumatic pillow 416 should besufficiently large so as to provide a force which exceeds the forcecaused by the mask, which is a product of the mask projected area(A_(mask)) and the mask pressure (P_(mask)). Hence:A _(bladder) ×P _(bladder)=ΣForces applied to strapsFurthermore,A _(bladder) ×P _(bladder) >A _(mask) ×P _(mask)

In one preferred form, the occipital pneumatic pillow 416 isapproximately 11 cm×16 cm and has wall thicknesses in the range of about1.5 mm to about 2.5 mm, with an overall deflated thickness of 3-5 mm.

In the embodiment described above, the pressure in the occipitalpneumatic pillow 416 increases when the mask pressure increases.However, in other embodiments of the invention, the inflation anddeflation of the occipital pneumatic pillow 416 could be controlled byparameters other than mask pressure. For example, a sensor could monitorleak in the mask assembly 412, e.g., by continuously monitoring flow inthe flow generator connected to the mask assembly 412 and low-passfiltering to find the leak component of the flow. When leak isdetermined to be high, the occipital pneumatic pillow 416 would becaused to inflate. Conversely, when leak is determined to be low, theoccipital pneumatic pillow 416 would be allowed to deflate. Controllingthe occipital pneumatic pillow 416 pressure using a leak detectionsensor would allow the headgear 414 to be maintained at the minimumamount of tension that would allow the mask assembly 412 to remainsealed against the face, and would help to reduce the user discomfort,skin damage, and other problems inherent in over tensioning the headgear414.

Additionally, it may be desirable to use more than one occipitalpneumatic pillow 416 in the headgear 414. If more than one occipitalpneumatic pillow 416 is used, the occipital pneumatic pillows 416 couldbe placed in several locations around the headgear 414. Moreover, eachof the multiple occipital pneumatic pillows 416 could be inflated anddeflated independently of the others. That type of arrangement wouldmake it easier to compensate for asymmetries in the patient's face,because tension could be applied in the headgear 414 locally and onlywhere needed. Multiple occipital pneumatic pillows 416 may be caused toinflate and deflate as pressure in the mask assembly 412 increases anddecreases, respectively, or they may be caused to inflate and deflate bya sensing and control system, based on measurements of leak flow.

In an alternative embodiment of the invention (as schematically shown inFIG. 82), shape memory alloy (SMA) wires, such as MUSCLE WIRES®(Mondo-Tronics, Inc., San Rafael, Calif., USA), which contract whenelectric current is applied, may be used as active tensioning elements.(Typically, the contractile response when electric current is applied isdue to heating of the wire caused by the passage of the electric currentthrough it.) If these types of elements are used to produce activetension adjustment, a separate controller would need to be provided tocause the wires to contract synchronously with increases in maskpressure.

Other suitable active tensioning elements include servo motors and“artificial muscles” created from biomimetic materials.

D. Algorithm

The occipital pneumatic pillow 416 according to the embodiment of FIG.16, e.g., can be initially inflated to a pre-set tension. In oneembodiment, a method for holding a mask sealingly against a patient'sface may include: placing an occipital pneumatic pillow against the backof the head and/or neck; passing one or more straps over, through or aspart of the occipital pneumatic pillow, the straps passing forward toattach to the mask; and inflating the occipital pneumatic pillow with apressure P_(bladder) which is an affine function of mask pressureP_(mask):P _(bladder) =P ₀ +A _(mask) P _(mask)where P₀ is a positive pressure sufficient to cause the mask to seal atthe lowest intended usage pressure, and A_(mask) is the lesser of thearea of contact between the occipital pneumatic pillow and the strapsposteriorly, and the area of contact between the occipital pneumaticpillow and the back of the head anteriorly.

In embodiments, the inflating of the occipital pneumatic pillow with apressure which is an affine function of mask pressure comprises:measuring mask pressure with a pressure transducer, to produce a signalproportional to mask pressure; applying the signal to an amplifier withadjustable gain an offset; applying the output of the amplifier to avoltage controllable pressure source; inflating the occipital pneumaticpillow with gas from said pressure source; adjusting the offset so thatthe mask seals at the lowest required pressure; and adjusting the gainso that the mask seals at the highest required pressure.

In embodiments, if the signal V_(pt) from the pressure transducer isV_(pt)=K_(pt) P_(mask), the controllable pressure source produces apressure P_(c)=K_(c) V_(c), the projection in the posterior direction ofthe contact area of the mask with the face is A_(mask), the projectionin the anterior direction of the area of contact of the straps with theposterior surface of the occipital pneumatic pillow is A_(bladder), andthe force required to produce a seal at zero pressure is F₀, then theamplifier produces an output voltage:V _(out) =F ₀ /A _(bladder) +A _(mask) /A _(bladder) K _(c) /K _(pt) V_(in).

In embodiments, inflating the occipital pneumatic pillow with a pressurewhich is an affine function of mask pressure may comprise: connectingthe mask via a first hose to a first cylinder containing a first piston,the first piston in turn being connected via a linkage to a secondpiston in a second cylinder, the second cylinder being connected via asecond hose to the occipital pneumatic pillow; and biasing said linkageso as to inflate the occipital pneumatic pillow sufficiently to causethe mask to seal at the lowest intended usage pressure.

In embodiments, bias may be provided by a spring and/or a weight.

An apparatus for holding a mask sealingly against a patient's face mayinclude a first set of extensible straps, passing from the back of thehead forwards to the mask, the straps being tightened sufficiently tohold said mask sealingly against the face at the lowest intended usagepressure; a second set of inextensible straps, again passing from theback of the head forwards to the mask, and lying over the first set; andan inflatable occipital pneumatic pillow placed at the back of the head,between the first and second set of straps, said occipital pneumaticpillow being in pneumatic communication with the air in the mask.

In operation, the first, extensible set of straps provides a fixed,constant force, independent of mask pressure, and the occipitalpneumatic pillow, acting via the second set of straps, provides a forcewhich is a linear function of mask pressure. The two forces addtogether, to provide a force which is an affine function of maskpressure. The optimum arrangement will be approximately when theanterior projection of the smaller of the area of contact by theoccipital pneumatic pillow onto the back of the head and the area ofcontact of the occipital pneumatic pillow onto the second set of strapsis the same as the posterior projection of the area of contact of themask on the face.

In another embodiment, an apparatus for holding a mask sealingly againsta patient's face may include a set of rigid straps, passing from theback of the head forwards to the mask; a semi-rigid, springy occipitalpneumatic pillow placed between said straps and the back of the head,the occipital pneumatic pillow having a non-zero internal separationbetween the anterior and posterior walls at atmospheric internalpressure; and a hose connecting the occipital pneumatic pillow to themask.

The occipital pneumatic pillow may be conveniently constructed of anelastomeric material such as silicone, latex, or polyurethane. Itsspringiness may be adjusted by filling it with a springy material suchas a foam of silicone, latex, polyurethane, and/or PVC, or with one ormore internal or external springs. A comfortable internal spring can becreated from a second, sealed air and/or fluid-filled elastomeric inneroccipital pneumatic pillow, smaller than the outer occipital pneumaticpillow.

Optimally, the antero-posterior separation between the anterior andposterior walls of the occipital pneumatic pillow at atmosphericinternal pressure should be about 2-4 cm, for preference 3 cm, to allowa reasonable range of neck movement without overly increasing ordecreasing the strap force, and to allow for a considerable compressionof the structures on the back of the head (hair, skin, fat, muscle), andof the mask cushion and facial tissues, as the mask pressure increases.

The combined stiffness of the occipital pneumatic pillow walls and anyfoam filling or springs should for preference be such that it produces aforce sufficient to cause the mask to seal at all intended pressureswhen the straps are tightened to about mid-travel, or about 1.5centimeters below the untensioned length. Typically the force will be ofthe order of 200-600 g, depending on the characteristics and fit of themask.

As strap tension increases at higher mask pressures, the mask cushionand the tissues at the back of the head will be compressed. This willcause the occipital pneumatic pillow to expand. Since the occipitalpneumatic pillow is preferably intentionally stiff, and has beencompressed by tightening the straps below their loose length in order toprovide the force F₀ required to seal at arbitrary low pressures, thespringiness of the occipital pneumatic pillow will provide less and lessforce as the occipital pneumatic pillow expands. This loss of theinitial spring recoil force should be compensated by using a occipitalpneumatic pillow with an area A_(bladder) which is suitably greater thanthe area of the mask A_(mask).

When the occipital pneumatic pillow is under pressure, and actingagainst the back of the head anteriorly and against the strapsposteriorly, there will be a portion with flat cross section in contactwith the straps, and a curved portion on either side, not in contactwith the straps. Only the area in contact with the straps, andspecifically the anterior projection of this area, will be generatinguseful strap tension. In practice, for a typical sized full adult facemask, for example, an bladder with dimensions around 17 cm side to side,11.5 cm top to bottom, and 3 cm thick is suitable. The projected area ofcontact with the straps will be about 15 cm side to side by 10 cm top tobottom, so that the bladder will act approximately like a piston of area150 cm². Of course, these are examples only that can be modifiedaccording to preference and/or application.

Frame A. Adjustable Frame 1. First Embodiment

FIG. 36 represents an embodiment of the present invention including aframe 170 which is provided with a patient interface in the form of acushion 172. The frame 170 is supported on the head of the patient usinga plurality of straps 180, which may be made of substantiallyinextensible material as described above. Each strap 180 may include athreaded portion at an end thereof that is adapted to receive a nut 185so that the patient can adjust the tension in the straps. A warpingstrap 175 made of substantially rigid material is provided to the frame170.

An adjustment mechanism 176 is provided for additional adjustability ofthe frame 170. In particular, the adjustment mechanism 176 includes anadjustment screw 190 which can be rotated to effect translating movementof a wedge 195. The wedge 195 can be moved along an imaginary axis 196that is aligned with the upper most head strap 180 provided at the apexof the frame 170. Rotation of the adjustment screw 190 causes movementof the wedge 195 against the inside surface of the warping strap 175. Inan alternative shown in FIG. 36A, the wedge 195 may be provided toengage the outer surface of the warping strap 175. In either case, ifthe wedge 195 is moved upward toward the top of the frame 170, the topsurface 197 of the wedge 195 is forced against the inside or outsidesurface of the warping strap 175, which causes the frame to bend aboutan axis which is substantially parallel to or coincident with theimaginary axis 196.

This bending causes the lateral portions of the frame 170 to pushagainst the sides of the cushion 172, thereby imposing a pinching forceon the sides of the patient's nose. The provision of adjustability inthe lower part of the frame 170 allows the mask to more readily adapt todifferent types of nose features. Of course, the adjustability could beprovided along the top or middle portions of the frame as well. Theadjustability allows for the patient to set the desired contacting forcefor a given pressure, and the frame may flex, pivot or bend toaccommodate changes in pressure so that the force applied to the face issubstantially constant.

2. Second Embodiment

FIG. 37 illustrates an embodiment similar to the embodiment of FIG. 36,but includes additional features. For example, FIG. 37 discloses anaperture 200 which is surrounded by a ring shape member, e.g., a ferrousring 210. As shown in FIG. 38, the aperture 200 is adapted to receive aconnector 215 which is similar to an elbow. The connector 215 includes afirst ferrous ring 220 which is adapted to magnetically couple with theferrous ring 210 of the frame 170. The connector 215 includes a secondferrous ring 225 which is adapted to magnetically couple with a swivelmember 230 via a magnetic ring 235 provided in the swivel 230.

FIG. 38 shows more details on the construction of the cushion. Inparticular, the cushion 172 includes a base member 240, e.g., foam,provided to the inside of the frame 170 and a membrane 245, e.g.,silicone, that is supported by the base portion 240.

In FIG. 37, the distance between the top surface 197 of the wedge 195and the inside surface of the warping strap 175 is exaggerated so as tomore easily view the individual components.

FIG. 39 shows a detailed view of the adjustment screw 190, the wedge 195and the frame 170. In particular, the adjustment screw 190 includes athreaded portion 250 and a disc portion 255 provided at the opposite endof a knob 260 of the adjustment screw 190. The frame 170 includes across slot 260 adapted to receive the disc 255. The frame 170 has alongitudinal slot 265 adapted to receive the threaded portion 250 of theadjustment screw 190. The frame 170 includes an additional frontal slot270 to receive a lower body portion 275 of the wedge 195. The frame 170includes an extension 271 having an upright member 272 that is spacedaway from the bottom edge 273 of the frame 170. The upright member 272includes a bearing 274, which may be threaded. The wedge 195 includes agroove 280 which engages with inside wall members 285 which define thelongitudinal slot 265. The wedge 195 includes a partially threadedportion 290. The partially threaded portion 290 is adapted to engagewith the threaded portion 250.

To assemble the adjustment mechanism, the threaded portion 250 and thedisc 255 are inserted into the longitudinal slot 265 and the lateralslot 260, respectively. The partially threaded portion 290 of the wedge195 is then dropped on top of the threaded portion 250, with the bodyportion 275 initially positioned between the upright member 272 and theend of the inside wall members 285 adjacent the bottom end 273 of theframe 170. The groove 280 is guided to slide along wall members 285.Accordingly, upon rotation of knob 260, the wedge 195 will move back andforth within the channel 295 of frame 170. The extreme positions of thewedge 195 are shown in FIGS. 40A and 40B, respectively.

FIGS. 41 through 53 illustrate additional embodiments which allowpinching along the lateral sides of the patient's face/nose. Similarparts have been designated with like reference numbers as compared tothe embodiments of FIGS. 36 through 40B. Each of these embodimentsallows for more control over adjustment and adapts more readily tovariations in physiognomy between patient's with differently shapednoses.

3. Third Embodiment

In the embodiment of FIG. 41, knob 190 is operatively coupled to one andpreferably a pair of racks 191 which move in opposite directions uponrotation of knob 190. The distal ends 193 of the racks 191 engage withlateral portions of the frame, e.g., the frame includes cammed surfacewhich progressively increase in thickness as the distal ends 193 aremoved laterally outwards, to allow bending, flexing and/or pivoting ofthe frame about an imaginary vertical axis, to thereby enhance pinchingagainst the sides of the patient's face/nose.

4. Fourth Embodiment

In the embodiment of FIGS. 42A and 42B, the membrane will assist insealing small leaks, therefore accuracy of adjustment is not critical.In addition, the embodiment of FIGS. 42A and 42B substantiallyeliminates lift off from the chin. FIGS. 42A and 42B also includeadditional adjustment points, thereby enabling the fit of the mask tothe patient to be even more finely tuned. For example, each knob 190 isoperatively connected with a portion 197 which is provided to thecushion 172.

5. Fifth Embodiment

In FIGS. 43A and 43B, the frame 170 is semi-rigid while the cushion 172is mounted to a flexible member 300. As the strap tension is increased,the spring action of member 300 on the cushion will help in the sealingagainst the sides of the patient's nose. For example, frame 170 may be arelatively stiff spring, compared to the stiffness of flexible member300. Therefore, a patient with a crocodile type face may rely onstiffness of flexible member 300, and patients with a panda-like facemay rely on the flexible member 300 lying flat against frame 170 whichis flexible, but relatively stiffer than the flexible member 300. Thisembodiment is not shown to include an adjustment knob, but could beadapted as such.

6. Sixth Embodiment

In the embodiment of FIGS. 44A-44C, rotation of adjustment screws 190cause a cam shaped surface 194 of the screw to engage protrusions 196 onthe cushion support, thereby pinching the cushion inwardly towards thepatient's nose.

7. Seventh Embodiment

FIGS. 45, 46A and 46B illustrate yet another embodiment of the inventionwhereby the frame includes side wing portions 305 which can pivot withrespect to the frame 170. In concept, this embodiment is similar to theembodiment shown in FIG. 1 in which the finger portions 115 are providedto pivot with respect to the chassis 95. FIG. 46A is an exploded view ofthe embodiment of FIG. 45, while FIG. 46B is an assembled view of theembodiment of FIG. 45. This embodiment, like other embodiments, allowsthe sides of the cushion to more readily conform to various patient'shaving differently shaped noses. This embodiment also automaticallyconforms to the face shape since the straps 180 are connected to eachside wing portion 305 of the frame 170. Lift off of the mask/cushionfrom the cheeks is reduced, and a more even pressure of the cushion onthe face can be achieved. This embodiment, like many of the otherembodiments described above also allows for a replaceable or disposablecushion.

8. Eighth Embodiment

FIGS. 45A and 45B show yet another embodiment, similar to the embodimentof FIG. 45, in which the side wing positions 305 can be hingedlyconnected in a number of predetermined positions, by clicking ordetenting action. FIG. 45B shows a partially exploded view including atongue 306 provided between two hinge portions on the side wing portion305. The frame 170 includes a plurality of grooves 307 adapted toreceive the tongue 306, to thereby allow adjustability in a plurality ofdiscrete positions.

FIGS. 47 and 48 show the progressive sealing positioning of side wingportions 305 on the nose N of a patient, as the tension in the straps isincreased.

9. Ninth Embodiment

FIGS. 49 and 50 illustrate yet another embodiment of an adjustmentmechanism which allows the side wing portions to move, e.g., pivot, withrespect to the chassis or central frame of the mask assembly. Theadjustment mechanism includes a plurality of holes 312 which can receivea pin provided on a flange 313 of the side wing portion 305. The hinge311 could be a living hinge, a pin, an integral pin, etc.

10. Tenth Embodiment

FIG. 51 is an exploded perspective view of a mask assembly 510 accordingto another embodiment of the invention. The mask assembly 510 has twomajor portions, a semi-rigid mask chassis (or shroud) 512 and acushion/secondary frame 514. The two portions 512, 514 are separable,but may be releasably or fixedly connected, as described above. Ingeneral, the mask chassis 512 is constructed and arranged to connect tomask headgear (not shown in FIG. 51), and the cushion/secondary frame514 is constructed and adapted to make a comfortable seal with apatient's face. The mask chassis 512 and the cushion/secondary frame 514have structures that cooperate to cause the cushion/secondary frame 514to move or deform relative to the mask chassis 512 so as to providesmall adjustments in the fit of the mask assembly 510 to the user'sface.

In the following description, certain directional terms, such as “top,”“bottom,” “left,” and “right” will be used. Unless otherwise indicted,the directional terms are used with respect to the coordinate systems ofthe respective drawing figures.

The cushion/secondary frame 514 comprises a cushion portion 516 (with aposterior opening configured to receive at least a patient's nose) and asecondary frame portion 518. The two portions 516, 518 are fixedlyconnected. The cushion/secondary frame 514 may be sized to act as amouth mask, nose mask, mouth-and-nose mask, or any other type of maskthat is compatible with the user's treatment protocol.

The secondary frame portion 518 is triangularly or pyramidally shapedand provides sufficient interior volume to accommodate the facialfeatures over which the cushion/secondary frame 514 is designed to makea seal (e.g., nose, nose and mouth, etc.). The secondary frame portion518 is open on two sides. On the outward side of the secondary frameportion 518, a connector 520 with an anterior aperture is provided toconnect to a gas supply conduit. On the inward side, the secondary frameportion 518 is open and flares into a flange 522, to which the cushionportion 516 is connected. The secondary frame portion 518 may be made ofa flexible or semi-flexible material, e.g., polypropylene.

The cushion portion 516 is a generally soft and conforming structurethat may be, for example, a silicone membrane, foamed material (such aspolyurethane foam) encapsulated within a plastic membrane, or a sealed,deformable compartment filled with air or another gas. It may be moldedto (i.e., fused to) the secondary frame portion 18, fixed usingadhesives, or secured with appropriate connecting structures.

The secondary frame portion 518 also includes structures constructed andarranged to connect the cushion/secondary frame 514 to the mask chassis512. At the top and bottom of the secondary frame portion on itspatient-outward surface are connecting members (or projections) 524 thatare adapted to be inserted into corresponding receiving holes (orrecesses) 526 in the mask chassis 512 to secure the cushion/secondaryframe 514 to the mask chassis 512. Connecting members 524 areconstructed and arranged to deflect inwardly on insertion into thereceiving holes 526 to provide a snap fit between the cushion/secondaryframe 514 and mask chassis 512. Although connecting members 524 areshown in FIG. 51, the connection between the cushion/secondary frame 514and the mask chassis 512 may be any other type of suitable connector.The secondary frame portion 518 also includes projections 528 withsurfaces that cooperate with adjustment wheels 530 in a manner that willbe described below.

The mask chassis 512 is a generally triangular contoured plate ofsemi-rigid material, which may be co-molded with the cushion/secondaryframe 514. The mask chassis 512 provides connecting receptacles 532 forcorresponding ends of the mask headgear 534. In mask chassis 512, twoconnecting receptacles 532 are provided, one at each of the left andright edges of the mask chassis 512. However, any number of connectingreceptacles 532 may be provided, disposed about the mask chassis 512 asrequired, depending on the number and position of the headgear straps orstrap ends. In FIG. 51, the connecting receptacles 532 and mask headgear534 are illustrated as having releasable snap-fit connections. However,the connecting receptacles 532 may be any type of conventionalconnecting structure. The top edge 540 of the mask chassis 512 (at anextension of the mask chassis 512 in the form of a forehead support witha stem portion) generally includes a connecting structure for connectingto a sagittal strap or strap portion of the mask headgear. Depending onthe configuration of the sagittal strap or strap end, the connectingstructure at the top edge 540 may be a connecting receptacle 532 oranother connecting structure.

The mask chassis 512 includes a central aperture 536 that is constructedand sized to receive the raised, central portion 538 of thecushion/secondary frame 514, such that the connector 520 may beconnected to an appropriate conduit for gas delivery through the centralaperture 536 of the mask chassis 512. Adjacent to the central aperture536 on the left and right sides of the mask chassis 512 are adjustmentwheel retaining portions 542. The positions of the adjustment wheelretaining portions 542 generally correspond to those of the projections528 on the cushion/secondary frame 514. Each adjustment wheel retainingportion 542 is raised relative to the surrounding surface of the maskchassis 512 and includes a hole, e.g., threaded hole 544.

The operation and interrelation of the adjustment wheel 530, adjustmentwheel retaining portion 542 and projection 528 are better illustrated inFIGS. 52 and 53, which are schematic cross-sectional views of a portionof the connected mask chassis 512 and cushion/secondary frame 514 in anengaged position, showing an adjustment wheel 530 installed in anadjustment wheel retaining portion 542 and engaging a projection 528 onthe secondary frame 518. The adjustment wheel 530 comprises a threadedrod or rivet 546, one end of which is secured into a user-turnable head548 to form a thumbscrew and the other end of which is coupled orprovided to the bore 544. In FIG. 52, the more narrow or thinner portionof the wheel 530 is in contact with the protrusion 528. Consequently,the cushion/secondary frame 514 is in a substantially undeformed statethat is not influenced by the mask chassis 512. The undeformed state ofthe cushion/secondary frame 514 may or may not make a good seal againsta user's skin, because of the presence of gaps or simply insufficientsealing force.

To adjust the force of the cushion/secondary frame 514 against the skin550, the user would turn the adjustment wheels 530, causing the thickerportion of the wheel 530 to move towards and contact the projection 528.Once the thicker portion has contacted the projection 528, the secondaryframe portion 514 deflects to thereby cause the cushion 516 to movetowards or more closely towards the patient's skin, thereby adjustingthe sealing force and/or fit. FIG. 53 illustrates the wheel 530 in aposition in which it causes the cushion/secondary frame 514 to deflecttowards the skin, eliminating the gaps and/or improving the fit/sealbetween the skin 550 and cushion portion 516.

In addition to the adjustment mechanism described above, certainportions of the mask chassis 512 may be locally weakened in order toallow the mask chassis 512 to flex slightly to accommodate variousfacial shapes. For example, the portions of the mask chassis 512 alonglines W may be locally weakened to allow the mask chassis 512 to flex.The mask chassis 512 could be made stiffer than the secondary frame,either through materials, e.g., polycarbonate, geometry, e.g.,stiffening ribs, constraints, e.g., tension from headgear, orcombinations thereof.

FIG. 51 illustrates an embodiment of the invention in which the maskchassis 512 and cushion/secondary frame 514 are separate pieces, and inwhich the movement of the cushion/secondary frame 514 relative to themask chassis 512 is caused primarily by deflection. However, in otherembodiments of the invention, positioning structures may be included inthe mask chassis and/or cushion/secondary frame in order to move thecushion/secondary frame relative to the mask chassis. Additionally, themask chassis and cushion/secondary frame may not be provided as separateor separable components, as described above in other embodiments.

B. Fins 1. First Embodiment

FIGS. 53A-53G illustrate two closely related embodiments of the presentinvention. Each embodiment is structured and arranged so as to helpimprove the lateral sealing force against the sides of the nose. Theseembodiments have particular use in the nasal bridge region, but couldalso be applied in other regions, such as the region adjacent the lowerportion of the nose or the mouth.

FIGS. 53A-53C illustrate one embodiment in which a frame 600 has agenerally triangular shape and in this example is structured to supporta full face mask cushion, although it could be adapted for use as aframe for a nasal cushion. The frame 600 may include other shapes, suchas generally round, trapezoidal, or any shape that accommodates theintended area of the patient which serves as the interface.

The frame 600 includes a pair of lateral members 605 each including aconnector interface 610 with at least one and preferably a plurality ofapertures 615. Each aperture 615 is structured to receive an end of aheadgear strap, preferably of the substantially inextensible typedescribed above. The headgear strap can be connected to any one of theapertures 615. Alternatively, the apertures can be provided to a sidewall 617 of the connector interface 610.

As shown in FIGS. 53B and 53C, a surface 616 oriented towards thepatient's face includes a pair of fins 620 that are positioned justlaterally outwards of the sides of the patient's nose in use. As seen inFIG. 53b , the fins 620 are positioned in the nasal bridge region of thepatient's nose, but the fins 620 could also extend along a greater or anentire extent of the sides of the patient's nose. The fins 620 arestructured to be received in a corresponding slot or groove formed inthe facial cushion (not shown) or on the outside of the cushion. Eachfin 620 may include one or more holes 625 which can help create a lockbetween the fin and the cushion, and may also reduce weight.

In use, the fins 620 provide a degree of lateral support to helpmaintain a good seal against the sides of the patient's nose. Forexample, as tension in the straps is increased, a normal cushion willhave a tendency to billow laterally outwards, thus increasing the chanceof compromising the seal or comfort of the cushion. The provision of thefins 620 helps prevent the cushion from billowing outwardly, to therebyhelp maintain the seal against the sides of the patient's nose.

The frame 600 may be structured to be flexible to as to be able topivot, bend or flex generally about an axis A. As such, when theheadgear straps are tightened, the frame 600 may move about the axis A,thereby causing the fins 620 to move inwardly to pinch the sides of thepatient's nose. In addition or in the alternative, as the mask sides arebent, pivoted or flexed as shown by the arrows in FIG. 53C, a topportion 601 of the frame 600 bends, pivots or flexes towards thepatient's nasal bridge region, as indicated by the arrow in FIG. 53B. Inthe initial position, the top portion 601 is bent away from the face ofthe patient. Accordingly, adjustment of the side straps may causeenhanced sealing in the nasal bridge region, thereby avoiding the needfor a top strap.

When the mask is flexed around the primary, vertical, axis A, the frame600 tends to straighten about the secondary, horizontal axis B. Thisresults in the top 601 of the frame 600 moving closer to the face, andthe cushion being pressed into the nasal bridge.

This works on the principal that when the frame is bent around one axis,the section of the frame that is already bent around another axis willundergo a much larger strain. Hence to minimize the strain energy in thematerial, it will straighten out the inbuilt bend, to move to a lowerstrain energy position. As the inbuilt bend is away from the face,straightening this will move that part of the mask closer to the face.

It can be seen that by varying the position and direction of the out ofplane bending, any desired part of the frame can be made to move in andout as desired as the frame is bent in one known plane.

Alternatively or in addition, the frame 600 may include one or morelines of weakness 630, e.g., a hinge such as a living hinge, built in totop and/or bottom portions of the frame 600. The lines of weakness 600will allow the frame 600 to more easily move about the axis A.

2. Second Embodiment

The embodiment of FIGS. 53D and 53E is very similar to the embodiment ofFIGS. 53A-C. The main difference is that the connector interface 610 inFIGS. 53D and 53D includes only one aperture 615 for receiving an end ofa headgear strap. The aperture 615 includes a slot 617 which is sized toallow the relatively thinner portion of the strap to slide therein, butwill not allow the adjustable nut provided to the strap to pass.Further, as seen in FIG. 53E, the fin 620 does not include an apertureas in the embodiment of FIGS. 53A-C.

FIG. 53F shows the frame 600 of FIGS. 53D-E in an operative position ona model of a patient's head. The cushion C and elbow E have beenattached to the frame 600. FIG. 53G is a cross-sectional view of theframe.

C. Frame with Pad

FIG. 53H shows a further embodiment of the invention in which a frame640 is provided with a pad 645. The pad 645 extends along a bottomportion 650 of the frame 640 and may be provided below clip 655 and atleast a portion of headgear straps 656 that are provided to each lateralside 660 of the frame 640. The pad 645 may therefore provide additionalcomfort against possible abrasion of the clip 655 and/or strap 656against the patient's cheeks. The pad 645 may be made of a gel and/orfoam material.

D. Frame with Pegs

FIG. 53I illustrates a first frame member 661 and a second frame member662 carrying a facial interface, e.g., a cushion 663. The first framemember 661 includes a main opening 664 and a plurality of first holes665. The second frame member 662 includes a plurality of second holes666 that align with the holes 665 of the first frame member 662. Thesecond frame member 662 includes a protrusion 667 structured to beinserted through the main opening 665 of the first frame member 661.

A plurality of pegs 668 are provided between the first and second framemembers 661, 662. Each peg 668 includes a first end 669 and a second end670. The first end 669 is inserted into the first holes 665 while thesecond end 670 is provided to the second holes 666. Each peg 668 may beprovided with a spacer or stopper 671. Each peg 668 has a length “L”that is selected to adjust its depth in relation to the interior of thecushion 663. For example, the pegs 668 are shown to have various lengthssuch that the second end 670 penetrates the second hole 666 to a depthwithin the cushion 663 that is tailored to the patient's facialphysiognomy. As illustrated, the pegs 668 may hold the frames 661, 662and/or the cushion 663 in place via press fit retention or the pegs 668may include other structure to retain the cushion or frame members,e.g., barbs or undercuts.

E. Inflatable Cushion

FIGS. 53J-53P illustrate an embodiment of the invention where a frame170 supports a cushion 172 made, for example of silicone and/or foam.The frame 170 may include one or more headgear connection points 171provided about the perimeter of the frame 170. An exemplary headgearconnection point 171 may include a first lateral side wall and a secondlateral side wall. The first lateral side wall may at least partly forma border of a first opening configured to receive a headgear strap. Thesecond lateral side wall may at least partly form a border of a secondopening configured to receive at least one headgear strap. The cushion172 may include an inflatable bladder 173 provided in the nasal bridgeregion of the patient's nose. The bladder 173 is shown to be a singlepiece which is embedded within the cushion 172, but it could includeseparate pieces which are provided to discrete portions of the cushion172, in the desired positions. For example, the bladder may include twobladders which are provided to each side of the cushion, not includingthe apex of the cushion. The or each bladder 173 may be in communicationwith a source of material, e.g., air or gel, etc., to adjust the volumeof the bladder 173. The or each bladder is provided to reduce orincrease the distance between the associated cushion/frame section andthe patient's face. For example, FIGS. 53L-53P show the distance D inthe nasal bridge region being progressively decreased as the frame 170is pinched in at the sides. FIGS. 53L-53P show that the change in theshape of the sides of the cushion 172 is asymmetrical, although it maybe preferable to change the shape of the cushion and/or bladder 173 in asymmetrical manner, depending on whether the patient's nasal physiognomyis symmetrical or asymmetrical.

The cushion 172 can also be structured so as to minimize billowing ofthe sides of the cushion upon application of increased pressure. Forexample, the outer side walls of the cushion (remote from the patient'snose) can be made of a relatively thick gauge, so as to be relativelyimpervious to increased pressure, thereby reducing the chance that theouter wall will billow with increased pressure. Conversely, the innerwalls of the cushion (adjacent the patient's nose) can be made of arelatively thin gauge wall member, which will allow them to easilydeflect towards the patient's nose, thereby enhancing the seal.

Cushion

As used in this specification, the terms “rear” and “rearward” refer tothe side of the cushion assembly adapted to contact the wearer's faceand the terms “front” and “forward” refer to the side of the cushionassembly adapted to contact the mask shell or body. As also used in thisspecification, the term “mask” refers to nasal masks and full facemasks.

FIGS. 54A-54C are exploded rear elevation, side elevation and bottomplan views, respectively, of another known mask, the ACLAIM nasal mask,manufactured by Fisher & Paykel (F&P). The ACLAIM nasal mask includes arigid frame or shell 50 and a cushion assembly including a thin siliconeseal-forming membrane 10 and a foam rim 30. The shell 50 includes achannel 51 defined by an inner wall 52. In use the foam rim 30 ispartially positioned in the channel 51 and extends rearwardly (i.e.toward the wearer's face). The membrane 10 is secured to the edge of theshell 50 via a tongue and groove mechanism 60 (FIG. 54D). The foam rim30 serves as a supporting structure.

As used throughout this specification, the term “ACLAIM cushion” refersto the cushion assembly illustrated in FIGS. 54A-54D.

A problem with some prior art cushions such as the ACLAIM cushion isthat they can collapse under high pressures leading to the face beingsubject to the edge of the frame. This is uncomfortable for the patientand may result in marks or sores on their face.

Referring to FIGS. 55A-55D, a cushion assembly 100 in accordance with afirst embodiment of the present invention includes a silicone membrane110 and an undercushion (U/C) 120, similar to that of a MIRAGE® cushion.The membrane 110 and the undercushion 120 are supported by an underlyingcushion flange 140. The membrane 110 and the undercushion 120 may beformed as a one piece element with the underlying cushion flange 140.The membrane 110, the undercushion 120 and the underlying cushion flange140 may be formed of silicone and may be formed as separate elements, oras a one-piece unit. Although not shown in the figure, the cushionflange, the membrane and the undercushion are generally triangularlyshaped to match the contours of a wearer's face defined by the nasalbridge region, the cheek regions, the upper lip region (in the case of anasal mask), or the chin region (in the case of a full face mask). Asshown in FIG. 55D, the membrane 110 is generally the same shape as theundercushion 120 and surrounds the undercushion 120.

The cushion assembly 100 includes a flexible element 130 between themembrane 110 and the undercushion 120. In a preferred embodiment, theflexible element 130 is a foam insert. Referring to FIG. 55D, theflexible element 130 is placed between the membrane 110 and theundercushion 120. The flexible element 130 is supported by theundercushion 120 and provides initial soft compression of the cushionassembly 100. A flexible element in accordance with the first embodimentof the present invention is shown in FIGS. 56A-56F.

The flexible element 130 may be an insert constructed from a soft,compressible elastomer such as polyurethane foam. The flexible element130 may also be constructed from a soft silicone, for example with ahardness of Shore A 20 or less. The flexible element 130 may further bea thermoplastic elastomer.

The flexible element 130 acts like a spring exhibiting an initial lowspring constant. In addition to the flexible element 130 theundercushion 120 exhibits the characteristic of a relatively harder, orfirmer, spring constant. The respective roles of each layer in the firstembodiment are: (i) for the membrane 110 to cause a seal to occurbetween its outer surface and the user's face; (ii) for the flexibleelement 130 to serve as a compliance layer thereby preventing prematurecollapse of the membrane 110 onto the undercushion 120; and (iii) forthe cushion flange 140 of the cushion assembly 100 to serve as a supportlayer preventing excessive movement of the membrane 110 relative to theface thereby preventing the face from contacting a frame, body or shellof the mask or otherwise moving relative to the membrane 110 such as tocompromise the seal.

The flexible element 130 is shaped to track the cavity between themembrane 110 and the undercushion 120 of the cushion assembly 100. Thespacing of the flexible element 130 is relative to the membrane innersurface 115 such that there is still capacity for the membrane 110 tobillow outwards to seal against the patient's face.

FIGS. 57A-57C illustrate mechanical properties of the cushion assembly100 (FIG. 57A) in comparison to the mechanical properties of the MIRAGE®cushion (FIG. 57B) and the ACLAIM cushion (FIG. 57C). The y-axisrepresents relative cushion height h/ho. When the cushion assembly 100is at its original height ho, the relative cushion height is 1. If thecushion assembly 100 is compressed to half its original height, therelative cushion height would be 0.5. The x-axis represents thecompressive force F on the cushion assembly 100. The force F may be theresultant force on the cushion assembly 100 being used between the maskshell and a patient's face.

Referring to FIGS. 58 and 59A-59F, a cushion assembly 200 according to asecond embodiment of the present invention includes a membrane 210, aflexible undercushion 220, a flexible element 230, and an underlyingcushion flange 240. The membrane 210 and the undercushion 220 may beformed as a one-piece unit with the underlying cushion flange 240. Theunderlying cushion flange 240 is attachable to a mask frame or shell 250at a rear edge of the underlying cushion flange 240. In a preferredembodiment, the flexible element 230 is a foam insert. The flexibleelement 230 may also be formed of silicone.

FIGS. 60A-60C illustrate mechanical properties of the cushion assembly200 in accordance with the second embodiment of the invention, theMIRAGE® cushion and the ACLAIM cushion, respectively. FIG. 60Dillustrates a comparison of the three cushions on a single set of axes.The y-axis represents the relative cushion height h/ho and the x-axisrepresents the compressive force F on the cushion. The compressiveforces are represented as a percent of a maximum compressive forceapplied to the cushion assembly.

Referring to FIG. 60A, in the initial zone, which largely corresponds tothe membrane 210, the cushion height is reduced to about 95% by acompressive force of about 20% of the maximum force. In the second zone,which corresponds to the flexible element 230 of the cushion assembly200, compression goes from 95% to 80% by a compressive force of about60% of the maximum force. In the third zone, which corresponds to theundercushion 220, increasing the force up to 100% of maximum onlyslightly further compresses the cushion assembly 200.

FIG. 60B illustrates mechanical properties of a MIRAGE® cushion.Compression of about 75% is achieved by a force of about 50% of maximumin the zone corresponding to the membrane. Further increases in thecompressive force results in only slight decreases in the cushionrelative height, corresponding to compression of the undercushion.

FIG. 60C illustrates mechanical properties of an ACLAIM cushionassembly. An initial compression to about 90% is achieved relativelyeasily with low force. Thereafter, further increases in the compressiveforce lead to large compression of the foam insert by about 30% of themaximum force. Application of a compressive force above 30% of maximumresults in very slight, or almost no compression of the ACLAIM cushionassembly as the foam insert is completely compressed against the maskshell.

FIG. 60D illustrates a comparison of the three previous curves on oneset of axes.

FIG. 61 is a graphical representation of the functioning of the cushionassemblies 100, 200 according to the first and second embodiments byanalogizing the flexible element 130, 230 and the undercushion 120, 220,respectively, to springs. The spring constant of the flexible element130, 230 is smaller than the spring constant of the undercushion 120,220. As shown in FIG. 61, the flexible element 130, 230 acts as a softspring to initially take up a compressive force on the cushion assembly.The flexible element 130, 230 enhances the soft feel of the cushionassembly 100, 200 and enhances conformance of the cushion assembly 100,200 to the wearer's face, thus improving the seal and reducing, oreliminating, leaks.

As the compressive force on the cushion assembly 100, 200 increases, thestiffer undercushion 120, 220 subsequently begins to compress. Theundercushion 120, 220 reduces, or eliminates, the possibility of thewearer's face from contacting the mask shell.

Referring to FIG. 62, a cushion assembly 300 according to a thirdembodiment of the present invention includes a membrane 310 that extendsfrom an underlying cushion flange 340. The underlying cushion flange 340is attached at a rear end thereof to a mask shell 350. A flexibleelement 330 is disposed between the membrane 310 and the underlyingcushion flange 340. The flexible element 330 of this embodiment isgenerally taller, or deeper, than the flexible elements of the first andsecond embodiments. The flexible element 330 in this embodiment is madeof one material. The cushion flange 340 does not include anundercushion. In this embodiment, the membrane 310 achieves the primaryseal and is supported by the flexible element 330, which distributes thecompressive forces at various locations resulting in a more comfortablemask system.

Further views of the flexible element 330 are shown in FIGS. 63A-63E. Inthis embodiment, the flexible element 330 includes multiple segments331, 332, 334, 335 of different properties (force deflectioncharacteristics). As shown in FIG. 63B, which illustrates the rear sideof the flexible element 330, the segments 333 and 334 may be placed inthe nasal bridge region of the mask. The segments 333 and 334 may be ofdifferent sizes and shapes and different spring constants (i.e.stiffnesses) to accommodate differences in sizes and shapes ofindividual wearers. It should be appreciated that the segments 331, 332,333, 334 may be of varying sizes, shapes and spring constants toaccommodate differences in the sizes and shapes of individual wearers.It should also be appreciated that any number of segments may be used.

The flexible element 330 is supported by a rigid retainer 355. Theretainer 355 holds the flexible element 330 during assembly of thecushion assembly 300 and the mask. The flexible element 330 and theretainer 355 may be joined together and used as a sub-assembly. Theretainer 355 may also include an extension having an elbow retainer clip360. The elbow retainer clip 360 retains a rotating elbow 370 requiredfor the supply of air from a flow generator. Further views of theretainer are shown in FIG. 64A-64E.

Referring to FIG. 65, a cushion assembly 400 according to a fourthembodiment of the present invention includes a two or three layerstructure. The first layer is composed of a cushion flange 440. A frontside of the cushion flange 440 is adapted to engage with a shell of amask. The second layer is composed of flexible element 430 connected tothe cushion flange 440. The flexible element 430 defines a cushion shapeand forms a face-engaging structure. The flexible element 430 may beprovided with a third layer composed of a skin 460.

The first layer (cushion flange 440) may be a rigid layer ofpolyurethane elastomer (no foam). The second layer (flexible element430) may be formed of urethane foam or soft silicone. The third layer(skin 460) may be formed of silicone skin. Preferably, the skin 460would have a thickness of 0.2 to 0.6 mm either uniform or varyingaccording to load or the expected degree of deformation required.

Referring to FIG. 66, a cushion assembly 500 according to a fifthembodiment of the invention includes a two or three layer structure. Thefirst layer is composed of a cushion flange 540. A front side of thecushion flange 540 is adapted to engage with a shell of a mask. Thecushion flange 540 includes a supporting portion 545 that extends fromthe front side towards the rear side. The supporting portion is flexibleand acts in a manner similar to the undercushion described above. Thesecond layer is composed of a flexible element 530 that is attached tothe cushion flange 540 and surrounds the supporting portion 545. Theflexible element 530 defines a cushion shape and forms a face-engagingstructure. The flexible element 530 may be provided with a third layercomposed of a skin 560.

The first layer (cushion flange 540) may be a rigid polyurethaneelastomer (no foam). The second layer (flexible element 530) may beformed of urethane foam or soft silicone. The third layer (skin 560) maybe formed of silicone skin with a uniform or varying thickness asdescribed above.

In use, the flexible element 530 begins to compress upon contact withthe wearer's face and application of a compressive force. As thecompressive force increases, the flexible element 530 is furthercompressed until completely compressed against the supporting portion545. Further application of compressive force results in compression ofthe supporting portion 545. The flexible element 530 and the supportingportion thus act as two springs in a manner similar to that illustratedin FIG. 61.

Referring to FIG. 67, a cushion assembly 600 according to a sixthembodiment of the present invention includes a first layer and a secondlayer. The first layer is composed of a cushion flange 640. A front sideof the cushion flange 640 is adapted to engage with a shell of a mask.The second layer is composed of flexible element 630 connected to thecushion flange 640. The flexible element 430 defines a cushion shape andforms a face-engaging structure.

The first layer (cushion flange 640) may be a rigid polyurethaneelastomer (no foam). The second layer (flexible element 630) is formedof foam and includes a first portion 631 of relatively low stiffness anda second portion 632 of relatively high stiffness. The differentstiffnesses may be provided by forming the flexible element of foamhaving different densities, as indicated by the relative spacing of dotsin the first and second portions 631 and 632. The flexible element 630may be a single piece having varying density, or multiple pieces withdifferent densities. Although the flexible element 630 is shown in FIG.67 as having two different stiffnesses (densities), it should beappreciated that the flexible element 630 may be formed with more thantwo stiffnesses (densities). As shown in FIG. 67, the second portion 632includes a region 632 a that extends beyond the cushion flange 640 toprevent the wearer's face from pressing up against the cushion flange640.

In use, the first portion 631 of the flexible element 630 begins tocompress upon contact with the wearer's face and application of acompressive force. As the compressive force increases, the first portion631 may be completely compressed whereupon the second portion 632 willbegin to compress. As the second portion 632 is stiffer than the firstportion 631, the decrease in the relative height of the cushion assembly600 caused by application of the compressive force will be less in thesecond region 632 than in the first region 631.

Although not shown in FIG. 67, it should be appreciated that the cushionassembly 600 may be provided with a third layer, such as a skin.

FIGS. 68 and 69 illustrate another embodiment of the present invention.A foam cushion 700 is provided to a frame 705. The frame includes anaperture 710 through which a support rod 715 is inserted. The aperture710 and support rod 715 may be threaded with one another such that theposition of a head portion 720 of the rod 715 may be moved as indicatedby the double ended arrow. As shown in FIG. 69, the frame 705 may beprovided with a knob 725 to allow movement of the rod 715. Although theframe 705 is only shown to include one adjustment rod 715, multiple sucharrangements could of course be provided. Adjustment of the position ofthe rod 715 allows the cushion topography to be modified for a givenpressure range.

FIGS. 70-79 illustrate cross-sectional views of additional embodimentsof foam cushions according to the present invention.

In FIGS. 70 and 71, the interior of the foam cushion 729 includes aspring element 730. The amount of cushion material over the springelement 730 can vary depending on the location of the cushion inrelation to the patient's face. For example, in problem or sensitiveseal areas, the cushion can be modified such that the spring element 730is embedded more deeply into the cushion, whereby a seal with theproblem or sensitive area is formed by the cushion alone, substantiallywithout much, if any, contribution of spring element. One advantage isthat the spring element 730 is fully embedded and invisible, so thatthere is a perceived simplicity, which can effect therapy compliance ofthe patient.

FIG. 72 includes a cushion 732 made of foam and a bellows 734,preferably made of silicone, provided to an inside surface of thecushion 732. The bellows 734 can readily conform to the patient's face.FIG. 73 is an arrangement which has a membrane 736 on the outside of thesupport 732.

FIG. 74 is an arrangement in which a cushion 732 is provided with achannel 738 in communication with a chamber 740 arranged internally ofthe cushion 732. The channel 738 is in communication with a source ofpressurized air or other medium, e.g., gel, such that the stiffnesscharacteristics of the cushion can be changed.

FIGS. 75A, 75B and 79 illustrate various cross sections which provide atleast two spring rates as the cushion is compressed. In FIGS. 77 and 78,the cushion 732 includes at least three spring rates during compression.In FIG. 77, the cushion 732 includes three layers L1, L2 and L3, each ofwhich has a different spring constant. FIG. 78 will have a first springrate upon compressing head portion 747 into first groove 748, a secondspring rate upon compressing central portion 749 into second groove 751,and a third spring rate upon engaging base portion 753.

FIG. 76 illustrates a cushion 732 with a dimple 756 which may helpmaintain the seal, e.g., via suction, between the cushion and thepatient's face.

An advantage of a foam insert as the flexible element is that foam ismore compressible than silicone and thus a cushion assembly including afoam insert in accordance with the present invention achieves a betterdistribution of the load from the headgear.

An advantage of silicone as the flexible element is that silicone iseasy to clean and possibly more biocompatible with patients. Note thateach element, membrane, flexible element, and cushion flange, in theembodiments described above acts as a mechanical spring. The embodimentsdescribed above may be used in combination, for example, the flexibleelements of the first and second embodiments may have multiple regionsof differing stiffnesses (densities) or the individual segments of theflexible element of the second embodiment may have multiple regions ofdiffering stiffnesses (densities). Many combinations of the disclosedmembrane, flexible element and cushion flange can be used to achieve thedesired comfort level.

The flexible element can be made from viscoelastic foam of a constant ormultiple densities to provide the desirable effect. The flexible elementcan also be made from open or closed cell foam of constant or multipledensities. The flexible element may or may not be covered with a skin.

A mask assembly including a cushion assembly in accordance with anembodiment of the invention can use a four-strap headgear similar tothat of the ULTRA MIRAGE™ mask system, manufactured by ResMed Limited.

A mask assembly including a cushion assembly in accordance with anembodiment of the invention can use headgear clips in accordance withU.S. Pat. No. 6,374,826, the contents of which are hereby included bycross reference.

An advantage of the cushion assemblies of the present invention to amask system is increased comfort for the user. This is possible due touniform distribution of the forces for people who need substantial straptension to achieve a seal. The flexible element also helps maintain theshape of the membrane and an effective seal while the user moves duringthe night.

Other advantages of the cushion assemblies of the present inventioninclude ease of manufacturing and reduced manufacturing costs. Thecushion assemblies of the present invention are also less complicatedthan prior art cushions, for example, the ACLAIM cushion which is athree piece cushion requiring assembly before being affixed to the mask.

A further advantage of the present invention is that the flexibleelement results in less distortion of the membrane. This allows thephysician/clinician to fit the mask in less time with a more reliableseal.

A further advantage of the invention is that the provision of a flexibleelement under the membrane achieves a more stable seal. The flexibleelement provides a smaller variation in the leak rate between the faceand the cushion due to reduced mask movement. This enhances the therapyprovided by the bi-level machines.

To provide the correct force, the flexible element should haveappropriate force displacement characteristics. This requirement is thatthe element should be soft enough to deflect initially to match thefacial features but should not result in complete compression anddiscomfort to the user due to pressing of the rigid part of the cushionflange.

In some of the examples described above, the cushion may be provided tothe frame using adhesives. In another embodiment shown in FIG. 80, thecushion may be provided to the frame using a mechanical fastener. In theexample of FIG. 80, a mask assembly 900 includes a body portion 902 anda cushion 904 provided to the body portion 902. The cushion 904 mayinclude a neoprene cushion 906 covered with a continuous siliconmembrane 908, for example. A flexible portion 910 may be provided alongthe perimeter of the body portion 902, for reasons described above. Thebody portion 902 and the membrane 908 may be formed of a single piece inthis example. The membrane 908 includes an inner perimeter shoulder 912and an outer perimeter shoulder 914 to engage a surface 916 of theflexible frame 910. The outer perimeter shoulder 914 may wrap around theedge of the flexible frame 910. Alternatively, the outer perimetershoulder may be said to include a groove to accommodate the outer edgeof the flexible frame 910. Similarly, the inner peripheral shoulder mayinclude a groove to accommodate the inner perimeter of the flexibleframe 910. Stated differently, the member 908 “snaps on” to the edges ofthe frame 910.

Snapping the edge of the cushion against the frame removes the necessityto glue the neoprene cushion to the frame, although gluing may be usedbetween the neoprene cushion and the silicone membrane. This arrangementprovides superior integrity of the assembly providing a more reliableconnection between all the parts and also removes any problemsassociated with chemical compatibility between the neoprene cushion andthe nylon frame necessary for adhesion.

The mask assembly is robust enough to withstand forces exerted byrigorous use, thus resulting in greater reliability and patientsecurity. The membrane 908 provides a sealed air path which reduces airleak.

A relaxation curve for a foam suitable for use in the flexible elementaccording to the invention is shown in FIG. 81.

The following table lists exemplary properties of a foam suitable foruse in the flexible element according to all embodiments of the presentinvention:

Property Value Density 0.1752 (g/cm³) CLD @ 25% 1.054 (N/cm²) CLD @ 65%3.375 (N/cm²) Sag Factor 3.20 IHF 4.36 Recovery % 57

The definitions which are found below were obtained from publiclyavailable reference materials, and may not use the same units of measureas described above in relation to the above table of exemplaryproperties.

Density is the weight per unit volume of the foam and below expressed inpounds per cubic foot (pcf.), although the above density is measured ing/cm³. The general range of polyether flexible urethane foams is 1 to 4pcf. This density is not a measure of firmness as is the case with latexrubber foams. For a given load bearing requirement, the higher densityfoam generally gives better quality and performance.

Compression load deflection (CLD) is also a measure of load bearing andis generally expressed in pounds per square inch (psi) at a givenpercentage deflection. The entire sample is compressed in this test andthe values are independent of foam thickness, providing thickness doesnot exceed length and width measurements. CLD is used to specifyfirmness of certain types of specialty foams and some semi-flexiblefoams. Values also are used in determining changes in load bearing undervarious humid aging or heat aging conditions.

Indentation load deflection (ILD) is one measure of load bearing and isexpressed in pounds load per 50 square inches at a given percentagedeflection of the foam. To obtain the value, a 50 square inch circularplate (a) is pushed into the foam top surface, stopping at a givendeflection and reading a load or a scale. For example, a 25 percent ILDof 30 means that it takes 30 pounds load to compress a 4 inch thickpiece of foam to a 3 inch thickness. The higher the load, the firmer thefoam. In this test, the foam sample size is larger than the circularplate, generally 15 by 15 inches for slab foams.

Some specifications define ILD with other plate configurations anddimensions. ILD is sometimes referred to as RMA (Rubber ManufacturersAssociation) from the same measurement used for latex foams. Suggestedpractice for specifying flexible foam is: USU (Urethane Slab Uncored);USC (Urethane Slab Cored); UMU (Urethane Molded Uncored); UMC (UrethaneMolded Cored). Digits following this code specify the 25 percent ILD,such as USU—30 refers to uncored slab with a 25 percent ILD of 30.Original thickness of the foam must be specified as values are affectedby the original foam thickness. (See BASF Wyandotte Technical Advisory,“Effect of Foam Thickness on ILD”.)

Sag factor is the ratio of 65 percent ILD to 25 percent ILD and gives anindication of cushioning quality. A high value indicates resistance tobottoming out. Foams with low sag factors will often “bottom out” andgive inferior performance. Other terms for this number are SAG factorand modulus.

Initial hardness factor (IHF) is the ratio of 25 percent ILD to the 5percent ILD. This factor defines the surface feel. Supple or softsurface foams will have a high value while boardy or stiff surface foamswill have a low value. Another term for initial hardness factor iscomfort factor.

In measuring ILD, values are normally taken at 25 percent deflection, 65percent deflection and again at 25 percent reflection as the load isremoved. The value of this 25 percent deflection on release of the loaddivided by the original 25 percent deflection is the recovery andexpressed as a percentage. High recovery values are desired forcushioning applications while low recovery would be desired for shockabsorbing applications. Low recovery foams are sometimes referred to as“dead.”

Indentation residual gauge load (IRGL) is another measure of loadbearing and is expressed as inches at a given loading. The same 50square inch circular plate is used as for ILD but now the plate isweighted with a given load. Normal loadings are 25, 50, or 75 pounds.The IRGL value is in inches. The original thickness of the foam must beknown to make the values meaningful. This measure is frequently used inautomotive foam specifications. There is no ready correlation betweenILD and IRGL values.

Guide factor is the ratio of 25 percent ILD to density. This term isuseful in determining the relative firmness of foams with differentdensity. The closer the densities, the better the comparison. Whendensities are different, the foam with the highest guide factor has thecost advantage, but not necessarily the performance advantage. Anotherterm for guide factor is normalized ILD.

Indentation modulus (IM) is the load required to produce an indentationof 1 percent between the limits of 20 percent ILD and 40 percent ILD.The slope of this line depends on resistance of foam cell walls tobuckling.

Resilience is a measure of elasticity or springiness of foam. In thistest, a steel ball is dropped on the foam and the rebound is expressedas percent resilience. As with recovery, desirable values are dependenton application. With very soft foam, resilience can be misleadingbecause the foam bottoms out under the load of the ball. This gives lowresilience values even though the foam is very “lively” or elastic. Ballrebound is another term for this property.

Tensile strength is a measure of the amount of stress required to breakthe foam as it is pulled apart and is expresses in pounds per squareinch (psi). Tensile strength can be used as a control check for quality.One common test is the determination of change of tensile strength afterheat aging.

Elongation is generally measured at the same time as tensile strength isdetermined. It is a measurement of the extent to which the foam can bestretched before it breaks and is expressed as a percentage of originallength.

Tear strength is a measure of the force required to continue a tear in afoam after a split or break has been started and is expressed in poundsper lineal inch (pli or more commonly pi). This property is important indetermining suitability of foam in application where the material issewed, stapled or “hog-ringed.”

Compression set is a measure of the deformation of a foam after it hasbeen held compressed under controlled time and temperature conditions.The standard conditions are 22 hours at 158° F. In the test, foam iscompressed to a thickness given as a percentage of its originalthickness. Compression set is most commonly expressed as a percentage oforiginal compression.

Fatigue is a measurement of the loss in load bearing under simulatedservice conditions and is generally expressed as a percentage load loss.The two most common fatigue tests are static fatigue and roller shearfatigue.

In a static fatigue test, the foam is compressed to 25 percent of itsoriginal thickness for 17 hours at room temperature. ILD losses arecalculated as percentages of original values.

In the roller shear fatigue test, a roller, longer than the foam width,is rolled back and forth across the foam. The roller is mounted in anoff-set position to impart a shearing action. Tests vary in use ofconstant deflection settings or constant roller weights. Losses arecalculated in ILD or IRGL as specified in the test method.

Air flow is a measurement of the porosity or openness of foam expressedin cubic feet of air per minute (cfm). Air can be pulled through a foamby vacuum as specified in the ASTM procedure or blown through usingapparatus as described in the BASF Wyandotte Technical Advisories, “BWCPortable Air Flow Apparatus” and “BWC Portable Air FlowApparatus-Improved Model”.

There are many other foam properties and test procedures. Many of thesehave been developed with specific end uses in mind. Further definitionof terms and description of test methods can be found in ASTM StandardMethods D-1564 and D-2406 in specific foam specification sheets.

While the invention has been described in connection with what arepresently considered to be the most practical and preferred embodiments,it is to be understood that the invention is not to be limited to thedisclosed embodiments, but on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the present invention.

The invention claimed is:
 1. A patient interface assembly configured to deliver a flow of breathable gas at positive pressure to a patient's airways, the patient interface assembly comprising: a cushion and frame assembly comprising a cushion portion and a frame portion, the cushion portion being configured to sealingly engage at least a portion of the patient's nose, the frame portion comprising an anterior aperture that is configured to receive the flow of breathable gas, the frame portion being relatively more rigid compared to the cushion portion; headgear comprising a back portion and a plurality of headgear straps; and a shroud removably mounted on the frame portion by way of a snap-fit connection, the shroud comprising: a pair of elongated arms, each elongated arm comprising a semi-rigid plastic material that is substantially inextensible and semi-flexible; a headgear connector provided to a distal end of each elongated arm, each headgear connector being configured to releasably connect to a corresponding one of the plurality of headgear straps; and a pair of living hinges, each said living hinge being configured to allow an associated one of the elongated arms to pivot towards or away from the patient's head in use, wherein, in use, each living hinge is independently and dynamically adjustable via pivoting due to headgear strap tension, and each said elongated arm is dynamically adjustable via flexing due to said headgear strap tension.
 2. The patient interface assembly of claim 1, wherein the shroud is more rigid than the cushion portion of the cushion and frame assembly.
 3. The patient interface assembly of claim 1, wherein the frame portion of the cushion and frame assembly is more rigid than the cushion portion.
 4. The patient interface assembly of claim 1, wherein the frame portion and the shroud are configured so that a flow path of the breathable gas from a gas delivery conduit to a breathing chamber formed by the cushion and frame assembly is through a central aperture of the shroud and the anterior aperture of the frame portion.
 5. The patient interface assembly of claim 1, wherein a portion of an outward surface of the frame portion is configured to remain uncovered by the shroud when the shroud is mounted to the frame portion.
 6. The patient interface assembly of claim 1, wherein the frame portion comprises a flange surrounding the anterior aperture.
 7. The patient interface assembly of claim 1, wherein the shroud comprises a rib.
 8. The patient interface assembly of claim 1, further comprising an elbow, wherein the elbow comprises an anti-asphyxia valve (AAV), wherein the headgear further comprises a coronal strap configured to engage the patient's crown, wherein the shroud is more rigid than the cushion portion of the cushion and frame assembly, wherein the frame portion of the cushion and frame assembly is more rigid than the cushion portion, wherein the frame portion and the shroud are configured so that a flow path of the breathable gas from a gas delivery conduit to a breathing chamber formed by the cushion and frame assembly is through a central aperture of the shroud and the anterior aperture of the frame portion, wherein a portion of an outward surface of the frame portion is configured to remain uncovered by the shroud when the shroud is mounted to the frame portion, and wherein the patient interface assembly is in the form of a full-face mask.
 9. The patient interface assembly of claim 1, further comprising an elbow directly attached to the frame portion of the cushion and frame assembly, wherein the elbow comprises an anti-asphyxia valve (AAV), wherein the plurality of headgear straps includes at least one upper headgear strap and at least one lower headgear strap that are separately formed components attached to each other at joints, wherein the shroud is more rigid than the cushion portion of the cushion and frame assembly, wherein the frame portion of the cushion and frame assembly is more rigid than the cushion portion, the frame portion and the shroud are configured so that a flow path of the breathable gas from a gas delivery conduit to a breathing chamber formed by the cushion and frame assembly is through a central aperture of the shroud and the anterior aperture of the frame portion, wherein a central portion of an outward surface of the frame portion is configured to remain uncovered by the shroud when the shroud is mounted to the frame portion, wherein the headgear straps are configured to be looped, wherein the patient interface assembly is in the form of a full-face mask, wherein the frame portion comprises a flange surrounding the anterior aperture, and wherein the shroud comprises a rib.
 10. The patient interface assembly of claim 1, further comprising an elbow comprising a gas washout vent, wherein the shroud is more rigid than the cushion portion of the cushion and frame assembly, wherein the frame portion of the cushion and frame assembly is more rigid than the cushion portion, wherein the frame portion and the shroud are configured so that a flow path of the breathable gas from a gas delivery conduit to a breathing chamber formed by the cushion and frame assembly is through a central aperture of the shroud and the anterior aperture of the frame portion, and wherein the patient interface assembly is in the form of a nasal mask.
 11. The patient interface assembly of claim 1, wherein the headgear straps extend from the back portion.
 12. The patient interface assembly of claim 1, wherein an integrally formed extension of the shroud is in the form of a forehead support and comprises an anchored end, a free end with an upper headgear connector, and a stem portion between the anchored end and the upper headgear connector, the stem portion of the forehead support supporting the upper headgear connector so that the upper headgear connector is positionally fixed relative to the anchored end of the forehead support, wherein the forehead support is arranged to extend beyond the cushion and frame assembly when the shroud is mounted on the cushion and frame assembly, wherein the free end of the forehead support with the upper headgear connector is configured to intersect the patient's sagittal plane when the patient interface assembly is mounted on the patient's face, and wherein the upper headgear connector is configured to secure at least one headgear strap to the forehead support above the patient's eyes.
 13. The patient interface assembly of claim 12, further comprising an elbow configured to connect to an air delivery tube, the stem portion of the forehead support extending along a plane that extends through the elbow, wherein the forehead support comprises the top edge of the shroud and is formed from semi-rigid material, wherein the forehead support is removable with the shroud from the cushion and frame assembly, wherein the stem portion of the forehead support is configured to extend along the bridge of the patient's nose and along the patient's sagittal plane, wherein the free end of the forehead support is wider than the stem portion and comprises first and second openings configured to receive the at least one headgear strap, wherein the free end of the forehead support comprises a first lateral side wall and a second lateral side wall, the first lateral side wall at least partly forming a border of the first opening configured to receive at least one upper headgear strap, and the second lateral side wall at least partly forming a border of the second opening configured to receive the at least one headgear strap, and wherein the first and second openings that are configured to receive the at least one headgear strap are configured to be positioned on opposite sides of the patient's sagittal plane.
 14. The patient interface assembly of claim 1, wherein the living hinges are positioned between the elongated arms, and wherein at least a portion of each headgear connector is located superior to the living hinges.
 15. The patient interface assembly of claim 1, wherein at least one of the living hinges is configured to be below the patient's eyes during use.
 16. The patient interface assembly of claim 1, wherein the headgear comprises a pair of upper headgear straps connected to the shroud and a pair of lower headgear straps indirectly connected to the shroud by way of a pair of corresponding clips.
 17. The patient interface assembly of claim 1, wherein the cushion and frame assembly and the shroud comprise a projection and recess arrangement that is configured so that pressing the shroud toward an outer surface of the frame portion causes each recess to receive a respective one of the projections, and wherein one of the projections is located between a central axis of the anterior aperture and a superior apex of the shroud when the shroud is mounted onto the frame portion.
 18. A patient interface assembly configured to deliver a flow of breathable gas at positive pressure to a patient's airways, the patient interface assembly comprising: a cushion and frame assembly comprising a cushion portion and a frame portion that together form a breathing chamber, the cushion portion being configured to sealingly engage the patient's face and comprising a posterior opening configured to receive at least the patient's nose, the frame portion comprising an anterior aperture that is smaller than the posterior opening and is configured to receive the flow of breathable gas; headgear comprising a back portion and a plurality of headgear straps; and a shroud configured to be removably mounted on the frame portion by way of a snap-fit and comprising: a central region in the form of a contoured plate; a central aperture in the contoured plate; a pair of side wing portions, each side wing portion comprising an elongated arm portion comprising a semi-rigid plastic material that is substantially inextensible and flexible; a headgear connector provided to a distal end of each elongated arm portion, each headgear connector being configured to secure a corresponding one of the plurality of headgear straps to the shroud; and a pair of living hinges, each said living hinge being configured to allow an associated one of the elongated arm portions to pivot towards or away from the patient's head in use, wherein, in use, each living hinge is independently and dynamically adjustable via pivoting due to headgear strap tension, and wherein each said elongated arm portion is independently and dynamically adjustable via flexing due to said headgear strap tension.
 19. The patient interface assembly of claim 18, wherein the shroud is more rigid than the cushion portion of the cushion and frame assembly.
 20. The patient interface assembly of claim 18, wherein the frame portion of the cushion and frame assembly is more rigid than the cushion portion.
 21. The patient interface assembly of claim 18, wherein the anterior aperture of the frame portion and the central aperture of the shroud are configured so that a flow path of the breathable gas from a gas delivery conduit to the breathing chamber is through the central aperture of the shroud and the anterior aperture of the frame portion.
 22. The patient interface assembly of claim 18, wherein a portion of an outward surface of the frame portion is configured to remain uncovered by the shroud when the shroud is mounted to the frame portion.
 23. The patient interface assembly of claim 18, wherein the headgear straps are configured to be looped.
 24. The patient interface assembly of claim 18, wherein the frame portion comprises a flange surrounding the anterior aperture.
 25. The patient interface assembly of claim 18, wherein the shroud comprises a rib.
 26. The patient interface assembly of claim 18, wherein the headgear straps extend from the back portion.
 27. The patient interface assembly of claim 18, wherein the shroud comprises an upper headgear connector with at least one opening at a top edge of the shroud that is configured to receive at least one headgear strap, the upper headgear connector comprising a cantilevered stem portion configured to extend from a location below the patient's eyes to the at least one opening and being configured to support the at least one opening so that the at least one opening is positionally fixed relative to the rest of the shroud, wherein the stem portion is arranged to extend beyond the cushion and frame assembly when the shroud is mounted on the cushion and frame assembly, wherein the stem portion is configured to be aligned with and extend along the patient's sagittal plane when the patient interface assembly is mounted on the patient's face, and wherein the at least one opening is configured to secure at least one headgear strap to the upper headgear connector at a location above the patient's eyes.
 28. The patient interface assembly of claim 27, further comprising an elbow configured to connect to an air delivery tube, the stem portion extending along a plane that extends through the elbow wherein the stem portion comprises the top edge of the shroud and is formed from semi-rigid material, wherein the upper headgear connector is removable with the shroud from the cushion and frame assembly, wherein the stem portion is configured to extend along the bridge of the patient's nose and along the patient's sagittal plane, and wherein the at least one opening comprises a first opening and a second opening that are configured to be positioned on opposite sides of the patient's sagittal plane.
 29. The patient interface assembly of claim 18, wherein the living hinges are positioned between the elongated arm portions, and wherein at least a portion of each headgear connector is located superior to the living hinges.
 30. A patient interface assembly configured to deliver a flow of breathable gas at positive pressure to a patient's airways, the patient interface assembly comprising: a cushion and frame assembly comprising a cushion portion and a frame portion that together form a breathing chamber, the cushion portion being configured to sealingly engage the patient's face and comprising a posterior opening configured to receive at least the patient's nose, the frame portion comprising an anterior aperture that is configured to receive the flow of breathable gas; headgear comprising a back portion and a plurality of headgear straps; a shroud in the form of a contoured plate, the shroud being removably mounted on the frame portion by way of a snap-fit connection and comprising a central aperture; and a pair of side wings including a left side wing and a right side wing that are attached to laterally opposing sides of the contoured plate, each side wing comprising an elongated arm comprising a semi-rigid plastic material that is substantially inextensible and semi-flexible, each said elongated arm including a headgear connector at a distal end of the elongated arm, the headgear connector being configured to secure a corresponding one of the plurality of headgear straps to the shroud, wherein each side wing comprises a locally weakened region in the form of a living hinge that allows at least part of each side wing, including the respective elongated arm, to laterally pivot relative to the contoured plate, wherein the living hinge on the left side wing is adjustable independently from the living hinge on the right side wing, wherein, in use, each living hinge is dynamically adjustable via pivoting due to headgear strap tension, and each said elongated arm is dynamically adjustable via flexing due to said headgear strap tension, wherein each side wing is configured so that pivoting the side wing relative to the contoured plate moves the corresponding elongated arm with headgear connector toward or away from the patient's head in use, and wherein the cushion and frame assembly and the shroud comprise a projection and recess arrangement that is configured so that pressing the shroud toward an outer surface of the frame portion causes each recess to receive a respective one of the projections. 